A Gay Spike is an experience felt (mostly) by Queer people who wouldn't consider their gender to be related to masculinity or man/boyhood, or just someone who wouldn't normally consider themselves to like men in a gay way.
It basically describes when a person's sexuality and gender don't necessarily change, but feel even just slightly different so that their attraction feels Gay.
For example, a feminine-aligned nonbinary person could see a man somewhere, and suddenly feel a shift in their perception so that the way they see themself and their attraction is just different and more "gay" then what they would normally feel. It would also shift how their gender feels in some way that might adapt to the new feeling.
These spikes aren't a constant feeling, they just pop up.