Setup can be pretty standard: Ethan gives Giles a booby-trapped artifact. Zap across time. (Let's play the Game and assume that Holmes was a real historical personage in the Buffyverse.) Somebody steals the artifact from Giles. He needs it back so he can get back. Who else to turn to in this hopeless case but the specialist in hopeless cases?
Standard Doyle mode: Watson writing first person, so we get Giles first-hand from Watson, then second-hand in what Holmes chooses to say about him. Set it mid-1880s, pre-Reichenbach Falls. Giles can be a little thrilled to see Gilded Age London-- take in G&S at the Savoy, visit interesting locations destroyed in the Blitz, be appalled by the experience of the Tube, visit a tailor, become impatient with some social issue (probably to do with feminism, which Giles will take for granted). And of course he visits the Watchers of the time. (Interesting question to answer: why doesn't he go to the Watchers first instead of Holmes?)
Oh dear, this plot bunny has a twitchy little nose.