So, we were talking about steampunk last week, and I mentioned gaslamp fantasy in passing, but that's all. I figured now is as good a time as any to go ahead and cover it. So here we are. Steampunk and gaslamp fantasy are intensely tied together, and could almost be considered the same genre. Lord knows that they play into each other an awful lot. But where steampunk is a divergence of Victorian era technology, gaslamp fantasy is a divergence of the basic laws of nature. Just one. There's magic of some sort. Pretty obvious, yeah, but that's the main thing. There are more subtle differences, of course. Gaslamp fantasy doesn't rely as much on the subversive 'punk' nature of the main character(s), although it can be included. You're less likely to touch on how the tech works, for that matter.
Compared to other breeds of fantasy, gaslamp tends to be a lot heavier on the setting. It creates more of an ambience than a lot of its kinfolk. Take Jonathan Strange and Mr. Norrell, considered by many to be the prime example of gaslamp fantasy in literature (it's also a bit of a slog to get through, if you're wanting to read it. Fair warning.). There are pages of time spent just setting the scene and the history of the gaslamp fantasy world being created.
You see the same thing in His Dark Materials, which does jump in on the 'punk' part of things. A lot of time being put into setting the scene (though admittedly not as much as in Strange and Norrell). I guess you could say the same thing about contemporary (urban) fantasy, as well. The world is normally shown in a very descriptive vivid manner. In a way, gaslamp fantasy is the intersection between steampunk and contemporary fantasy.
I'd say that's the most obvious in the work that named the genre, Girl Genius. You have steampunk technology that can be used to create magic, but also unexplained things (a magic river, a strangely living castle, etc) that aren't specifically called magic. But they're magic in all but name. I'm not far in the comic, but I highly recommend it.
If you don't have time for that (it's not just Strange and Norrell: most gaslamp fantasy works tend to have some good heft to them, more than you would usually see in either steampunk or contemporary fantasy), but you're still interested, then look to Van Helsing. Much less technology-based than some gaslamp fantasy, it's very much Victorian era mixed with magic. Vampires and demons and holy power and werewolves. It straddles the line a bit, leaning heavily toward gothic horror, but it'll do for gaslamp fantasy in a pinch.
What other subgenres do you want to see? Let me know in the comments, and be sure to subscribe if you want to get the latest updates.