Run a deathmatch game, and type "sv_gravity" in the console to see the value of normal gravity.
Use a lower number than that one to reduce gravity, or a higher number to increase it.
It's not that the trigger_gravity can't be triggered again. Everytime it is triggered, it gives the client the same gravity value. If you want to change the gravity, you need the player to cross another trigger_gravity with a different value.
Take for example the shuttlebay in my map "Warpcore" There are two trigger_gravity in each door to the room, and over each spawn point inside. The inner ones give the client the low gravity, and the outer ones give the player normal gravity when they exit the room.