The term was coined by Kaja Foglio as a description of Girl Genius. The Foglios’ gaslamp fantasy universe is peopled with mad-scientist Sparks, their mechanical Clanks and biological Constructs, battling for world domination over common folks and to “Show them all!”.
I had been sorting through a lot of old sketches and noticed that Phil seemed to like drawing airships and weird creatures with walking sticks and old-fashioned clothes, but that stuff hardly ever made it into his final work. I've got a bit of a mania for that sort of thing myself, so it seemed a good idea to make a story that we could hang that look on. I called it Gaslamp Fantasy because, around the time we were bringing Girl Genius out, there was a comic called Steampunk on the shelves and I didn't want any confusion. Plus, I've never liked the term Steampunk much for our work, it's derived from Cyberpunk (a term which I think actually fits its genre well) but we have no punk, and we have more than just steam, and using a different name seemed appropriate. I mis-remembered a term that I had come across in the forward to an H. Rider Haggard book, where the author was talking about Jules Verne, H.G. Wells, Rider Haggard and that sort of pre-pulp adventure material, and came up with "Gaslamp Fantasy." I felt a bit foolish when I discovered that I had made up my own term, but it works and I like it.