One who spreads gayness. A gay fairy--much like someone with a yaoi paddle--has the ability to turn anything and anyone gay. Gay fairies are often also equipped with an insanely accurate gaydar. This is a rather peculiar phenomenon due to the fact that they have no need of gaydar. If someone is not gay, they'll make them gay just by being within a 5 mile radius of each other, so there's no point to it, really.
Gay fairies are also likely to be glitter fairies part-time.
If you find yourself suddenly interested in fanfiction, RP, rainbows, unicorns and/or pink see-through shirts, you very well may have recently been in the vicinity of a gay fairy.