Unlike steampunk, with its science fiction focus, gaslamp fantasy typically contains elements such as magic or creatures from mythology and folklore in a historical setting. The term is generally applied to the Victorian era, but works with the same elements that are set in that rough time period are included. Hence, some gaslamp fantasy may take place in the Regency era or shortly after the Victorian era. Works in an unspecified setting with similar cultural markers also exist.
While prominent, Hugo-winning 'Girl Genius' was the first work to use the term, plenty of other authors have written in the genre. Alan Moore's 'League of Extraordinary Gentlemen' comic series and film adaptation, features the era's literary figures such as Mina Murray and Captain Nemo. Jonathan Strange and Mr. Norrell by Susanna Clarke (Hugo winner for Best Novel in 2005), while taking place earlier, is also cited as gaslamp fantasy. The story revolves around a world in which magic is real and the two titular magicians' impacts on British history through their mischief. Many modern adaptations of stories like Dr. Jekyll and Mr. Hyde or Frankenstein with additional fantasy elements have a place in the genre as well. The Elemental Masters series by Mercedes Lackey features historical fantasy adaptations of fairy tales and classic stories, some of which fit into the time period. In particular, The Serpent's Shadow--a loose adaptation of 'Snow White' in 1800s London surrounding a doctor with earth powers--could be described as such.
Some children's and young adult novels contain gaslamp fantasy elements, like Etiquette and Espionage by Gail Carriger. Return of the Dapper Men by Janet Lee and Jim McCann and Garth Nix's Keys to the Kingdom septology come to mind also. However, these tend to spotlight the more popular steampunk tropes that young people will have seen in shows like Doctor Who and Wallace and Gromit. While the aforementioned shows aren't entirely 'steampunk', they feature visuals popularized by genre proponents that fans have come to associate with Victorian-inspired sci-fi. The Gemma Doyle books by Libba Bray are one of the few notable examples of YA gaslamp fantasy without that technological edge.
While this is a 'deep dive,' one really doesn't need to go 20,000 leagues under the sea to find great gaslamp fantasy.
For the beginner, I suggest looking into the excellent anthologies in the genre that feature short tales by big-name speculative fiction authors. Ghosts by Gaslight, edited by Jack Dann and Nick Gevers, features gaslamp fantasy and steampunk stories of all varieties. Notable authors include Robert Silverberg and Garth Nix and the collection highlights a mix of tropes and writing styles. Queen Victoria's Book of Spells, edited by Ellen Datlow and Terri Windling, exclusively contains gaslamp fantasy. There is also a greater focus on stories that feature witchcraft and social commentary on the era. There are many more, but these collections are a nice introduction.
The genre's existing and growing popularity makes it easy to find exemplars in literature, film and graphic works with a simple online search. While sharing shelf space and attention with steampunk, historical fantasy and time-travel historical fiction premises, it's in an (extraordinary) league of its own. Despite genre overlap, gaslamp fantasy retains its unique ability to sate fans' appetites for a world that never was--and often with a critical edge.