A novelty item invented by the infamous Dr. Prometheus Bunbury. Gil inadvertently uses one on Zeetha during their mock-duel outside Castle Heterodyne, with the result that her characteristic blue outfit is, well, destabilized.
This isn't Gil's real intention; he'd intended to use another of Bunbury's little toys, the "Jolly Fun Oxidation Enhancer," but pulled the wrong trinket out of his bag of tricks; apparently all of Bunbury's creations look more or less alike.
While Gil probably learned of such things during his student days in Paris, a careful examination of the strongly hints that the one he uses in the comic was looted from the supplies that Theo DuMedd assembled for his and Sleipnir O'Hara's proposed expedition to India. Why Theo felt the need to include this particular item in his equipment is left as an exercise to the reader.
Fortunately, Zeetha is prepared (semper ubi sub ubi) for this — apparently Bunbury has quite a reputation among actresses — with the result that Girl Genius' avoidance of NSFW issues is still intact, more or less, even if Zeetha's outfit isn't. (At least in the actual comic; in the print-novel adaptation, she isn't wearing leather underwear, but it doesn't slow her down at all. She eventually is given a robe by a helpful monk.)