This expression basically means your home-girlz. It is taken from the male origin man-dem. Can be used by both male and female homies, 2 describe thier home-gyal.
1) wassup gyal-dem? wot u up 2?
2) ive gotta gyal-dem thats obbsessed with that stuff! 4 real.
3) uve been my gyal-dems from pure time, trust.