Gothic Lolita is one of the three main Lolita styles (the others being sweet and classic). It is characterized as being related to western goth style and it has an overall dark and black look. Black is the most common main color for Gothic Lolita but white, jewel tones, silver and gold. Prints often contain gothic motifs such as crosses, castles, coffins, roses and bats. Platform shoes and high heels are used rather than teaparty shoes. Gothic Lolita used to be well known for its black with white lace look but after Gothic had been the most popular style for a while all the cheap badly made clothes that were trying to be lolita (horribly) started to try and copy that look and it lots all popularity due to being associated with this failed beginner look. I think the old school gothic look still hold up though if anyone wants to try and pull it of!
The biggest brands are Moi Meme Moite, h.Naoto, Atelier Pierot, Chantilly and Atelier Boz. Moi Meme Moite is the brand owned by gothic lolita fashion icon Mana, he is a famous visual kei artist that used to play in Malice Mizer and later in his own project Moi Dix Mois.
I made a collage to help you get the general feel of the style visually:
None of these are any rules of the style, just descriptions of whats common so that you can get an idea of what the style looks like, so don't be afraid to experiment!