does anyone know how to do this?

/noob spiel
++++answers, plz

Well i havent tried it before but i'm almost sure that it'll work if you put a trigger_push and set it to push the things upwards(pitch: 90). The speed of push: well i dunno i havent tried it, cuz im too lazy, but try some values.

i dont know the strenght of gravity in half-life units, but it should be weaker than that. Otherwise people will stuck to the ceiling:D

I'm not sure about this next bit:

env_gravity does not work in NS, since 2.0-ish (disabled, probs with buildings?) - ofcourse I could be completely wrong and it does work now...

trigger_push only works if a player has a z-axis speed (if you want up/down movement)

Meaning:

Downard pushing:
A skulk on a ceiling will just walk along until he jumps of the ceiling (then he gets a push)

Upward pushing:
Anything walking along that does not jump or has a z-speed will not be affected by it.

Trigger_push simply can't be used as env_gravity as it is quite jittery with it's movement and needs an initial speed to have any effect. Also any effect (large values) will have quite a large impact. I did make a sonic jumping spring once which worked quite effectively at catching falling people and pushing them into the air again without damage