Alternative 1:How about adding an upwards force, similar to those platforms that push you up (i.e. pushcannon) but instead of a platform to be a cubic area.
So it would work as
negative gravity thus decrease the sv_gravity from 800 to 500 or whatever you want.
English is not my native language so I find it a little bit hard to explain this physics stuff.
Am I being clear? Is it even possible through GTK?
Alternative 2:Add some kind of transparent water in which you don't die but there is less gravity. Although players will look as swimming and they'll be able to fly LOL
