Gothic metal is a type of metal which is fairly heavy, with heavy guitars, but what attributes most to gothic metal is the unique vocals, which make it gothic. If it's a male vocalist, the vocals will be low and deep, and in some gothic metal bands, there is no screaming/growling. If it's a female vocalist, then she just sings in her normal singing voice (no screaming, no POP voice - whining, moaning).
John said, "Hey what's that type of metal called when the dude sings really low, is it black metal or something?" . Micheal said, "No dude, it's gothic metal".