Is gothic a cult

If you're talking gothic as in the Goth / Gothic music scene then no. It's no different than any other scene (punk, metal, straightedge, hip-hop, electronic, folk). It's a group of like minded people that like a certain kind of music and dress a certain way and have certain subcultural traditions.

Goths are no more a part of a cult than kids listening to Hannah Montana are part of a cult.