Common terminology for an older male at central North American comic/anime/science conventions. A Gaelan usually "creeps" on young girls dressed in cosplay by following them around the convention centre or watching them from afar. A Gaelan is usually crossdressing and uses it to strike up a conversation. Tends to be very into roleplay; uses it in sexual ways.
S: "Hey Alannah who's your friend there...?"
A: "Oh just this guy I met, he's cosplaying Nana!"
(Pulls Alannah away so he can't hear)
S: "Dude he's a total Gaelan! Ditch him!"
G: "You... have epic cleavage."
S: "I'm only 16, gtfo!"