The Collins English Dictionary defines the informal, usually derogatory, British-English noun Guardianista as designating:

a reader of the Guardian newspaper, seen as being typically left-wing, liberal, and politically correct ¹.

¹ On Tuesday 18^th October 2022, the British Conservative politician Suella Braverman expressed the same idea in a different manner when she talked of “the Guardian-reading, tofu-eating wokerati”.

The noun Guardianista occurs, for example, in Mr Sunak, rising taxes and soaring prices do not add up to an Age of Optimism, by the British political journalist Andrew Rawnsley (born 1962), published in The Observer (London, England) of Sunday 31^st October 2021:

The prime minister and chancellor may be telling themselves that being attacked from both right and left demonstrates what a politically cunning budget this was. If they have angered Guardianistas while also provoking the wrath of Telegraphers ², they must be positioned in the centre ground, which is where most voters tend to congregate.

² The noun Telegrapher designates a reader of The Daily Telegraph (London, England), a newspaper which has been nicknamed (Daily) Torygraph because of its faithful adherence to the Tory Party, i.e., the British Conservative Party.

The noun Guardianista is from:
– The Guardian, the name of a centre-left newspaper published in London and Manchester, England;
– the suffix -ista, after nouns such as Sandinista ³ and Senderista ⁴.

³ The noun Sandinista designates a supporter of the Nicaraguan nationalist leader Augusto César Sandino (1893-1934), and a member of the revolutionary Nicaraguan guerrilla organisation founded by him or of a similar organisation founded in his name in 1963.
⁴ The noun Senderista designates a member of the revolutionary Peruvian guerrilla organisation Sendero Luminoso (i.e., Shining Path).
—Cf. also Corbynista.

The earliest occurrences of the noun Guardianista that I have found are as follows, in chronological order:

1-: From If you can’t stand the heat…, by the British food writer and critic Matthew Fort (born 1947), published in The Guardian (London and Manchester, England) of Saturday 27^th December 1997—here, the noun Guardianista (which designates both “a number of Guardian readers” and the author) seems to be merely humorous, without derogatory connotations:

I had the pleasure of escorting a number of Guardian readers on a cookery mini-break to Le Cheval Blanc in Blère in the Loire valley. The charm of my companions was at one with the delight of the food. The chef/prop of Le Cheval Blanc, Monsieur Bleriot, produced food of the utmost edibility, using butter and cream in quantities that are forbidden by dictat [sic] here. It was like stumbling across the exotic remains of some long-forgotten civilisation. We, fellow Guardianistas and I, fell on the food with cries of delight.

2-: From Perhaps he’s not so Dim Duisenberg after all, by the British economist Patrick Minford (born 1943), published in The Daily Telegraph (London, England) of Monday 30^th April 2001—here, the noun Guardianista has no obvious derogatory connotations, and may refer to writers in, rather than to readers of, The Guardian:

Shed a tear for poor old Wim Duisenberg ⁵. The Americans think he is out to lunch for daring to suggest that when the US catches flu, Europe will not sneeze much. The Guardianistas want him to safeguard the steady fall in Europe’s unemployment. Then—unkindest cut of all—the Belgian Finance Minister tried to get him sacked—fortunately Mr Trichet ⁶ is not available until his alibis have been confirmed in the Credit Lyonnais affair.
In short, everyone wants Wim to cut interest rates, or else. But dim fellow that he is, he thinks the Maastricht Treaty makes him and his European Central Bank independent.

⁵ The Dutch politician and economist Willem Frederik ‘Wim’ Duisenberg (1935-2005) was the President of the European Central Bank from June 1998 to October 2003.
⁶ The French economist Jean-Claude Trichet (born 1942), Governor of the Bank of France from September 1993 to November 2003, was accused of being party to signing off false accounts at Crédit Lyonnais, a major French bank.

3-: From an interview of the British journalist, author and broadcaster Richard Littlejohn (born 1954), by Nigel Farndale, published in The Sunday Telegraph (London, England) of Sunday 27^th May 2001; Richard Littlejohn was then a columnist for the British tabloid The Sun, and had just published the controversial novel To Hell in a Handcart—here, the noun Guardianista (which may refer to writers in, rather than to readers of, The Guardian) clearly has derogatory connotations:

Isn’t it dangerous for democracy to mock constantly those who govern us? ‘No it’s essential for democracy. Labour especially have far too many cheerleaders, sycophants and spin doctors. People go on about the power of the press but it is the politicians who f— people’s lives up.’ Littlejohn scratches his nose. ‘The Guardianistas monster me now because I’m ripping the arse out of the Labour Party, but not so long ago I was giving it to the Tories.’ […]
[…]
Richard Littlejohn is about to find out how he relates to another audience, the book-buying public. According to the blurb on the back of To Hell in a Handcart, ‘Richard Littlejohn exposes the madness of modern Britain in a thrill-packed roller coaster ride of a novel.’ ‘It’s not going to win a Booker Prize,’ he says, topping up his wineglass. ‘[…] If I could get the Guardian to write “racist, sexist, xenophobic, homophobic trash” about it I would put it on the cover as a recommendation.’

4-: From the review of Richard Littlejohn’s To Hell in a Handcart, by Stephen Moss, published in The Guardian (London and Manchester, England) of Thursday 14^th June 2001—here, the plural noun Guardianistas denotes “the liberal press”:

What are the liberal press, the “Guardianistas”, to make of Littlejohn? In the past, he has been given an easy ride, his highly personalised attacks on the Blairs and others he despises treated on his own terms—as “bottle throwing”. But when bottles are thrown, the broken glass can cause unforeseen damage, as David Aaronovitch pointed out this week in the Independent. He had no truck with the diamond geezer view of Littlejohn. Aaronovitch branded the book “racist” and described it as a “400-page recruiting pamphlet for the British National Party”.

In Crazy talk, “on how conflict throws up new phrases”, published in The Guardian (London and Manchester, England) of Wednesday 19^th September 2001, the British academic, author and newspaper columnist John Sutherland (born 1938) suggested that the noun ought to be Guardianisto:

In The Times, Michael Gove has devised the word “Guardianistas”, for the pantywaists (nervous nellies) who read this paper and have an occasional reservation about American foreign policy (“ethical”, Robin?—ah, happy days). It echoes the scornful label given the defeated corps of “Portillistas” ⁷ and, further back, the Zapatistas ⁸—the loyal (and victorious) band of revolutionaries who rode with Emilio Zapata. It should, of course, be “Guardianistos”—the Spanish is, I take it, short for “partida Zapatista”. But the imputation of effeminacy makes a point (who wears the panties in your house? man or mouse?). Me, I’m a Guardianisto and proud of it.

⁷ The noun Portillista designates a supporter of the British Conservative politician Michael Portillo (born 1953).
⁸ The noun Zapatista designates a supporter of the Mexican revolutionary Emiliano Zapata (1879-1919), and a member of the revolutionary guerrilla movement founded by Zapata, which fought during the Mexican Revolution to achieve the redistribution of agricultural land.

However, John Sutherland’s suggestion was dismissed as follows in Corrections and clarifications, published in The Guardian (London and Manchester, England) of Thursday 20^th September 2001—Corrections and clarifications is a column in which The Guardian highlights and reports its own errors:

A columnist, wrote as follows on page 6, G2, yesterday: “In the Times, Michael Gove has devised the word ‘Guardianistas’, for the pantywaists (nervous nellies) who read this paper…” The columnist saw in the word the imputation of effeminacy and said, “Me, I’m a Guardianisto…” In fact, there is nothing feminine or effeminate in Spanish -ista endings which indicate neuter nouns as in periodista (Spanish for journalist).