The phrase all Sir Garnet means: highly satisfactory; all right.

This phrase is from the name of Sir Garnet Wolseley (1833-1913), field-marshal and commander-in-chief of the British army, who conducted several successful military expeditions in the Sudan and elsewhere.

In Patience; or, Bunthorne’s Bride (1881), an operetta with music by Arthur Sullivan (1842-1900) and libretto by W. S. Gilbert (1836-1911), Colonel Calverley refers to:

The genius strategic of Cæsar or Hannibal
Skill of Sir Garnet in thrashing a cannibal.

The earliest use of the phrase that I have found is from an article titled A Contest of Voices (An Eastern Sketch), published in The St. James’s Gazette (London) of Tuesday 7^th March 1893, in which the reviewer of a musical spectacle wrote:

On the stage, whilst the black-haired pianiste is dreamily playing snatches of comic songs, comes one of the committee. We shall begin, moy [sic] friends, by Mrs. Norris. We have drawed lots, and Mrs. Norris is, as ye may sy [sic], number one. Therefore (says the committee-man, with the air of a stern logician)—therefore Mrs. Norris will sing first. […] Nᵒ. 1 goes off to much applause, and, after a wait, the committee-man puts his head round a wing decorated with eccentric flora, and shouts, “Nᵒ. 2. Mrs. James White will give us ‘The Rahdy Dahdy Boys.’” There is movement in the audience; a youth, smoking with the bowl of his pipe rakishly upside down, says approvingly to the befeathered damsel with him that this is all Sir Garnet, this is. Mrs. James White, in white with a large green sash that trails on the stage, is, indeed, quite a rollicking person. She is a little nervous at first, but the refrain puts her right, and she moves her arms, and is, I think, half inclined to kick.

Fond of a glars or two,
Fond of a rah or noise,
Hi-hi, clear the wy, for the Rahdy Dahdy Boys.

Nᵒ. 2 could have an encore if she chose to accept it. Instead she returns and bows awkwardly, and, turning, collides with the bustling committee-man—an incident which gives the hall ecstatic joy.