Andy's post doesn't answer his question, RM. What about other entities and players? Does the server change the gravity for everyone and everything or just the player?
Rat808: There really aren't any "rooms" or "zones" in a Half-Life map except those you create artificially. Andy's suggestion is that, by making sure you put two trigger_gravities back-to-back, you'll create a more believable "zone" of low gravity. It is not a "zone" that's created by the Half-Life engine because you put a trigger_gravity somewhere.
You could just as well put a single trigger_gravity in the middle of a "room" and have no way of returning gravity to normal. So what happens then?
If you notice, there are flags for the trigger_gravity entity: Monsters, No clients and Pushables.
The implication is that you can make the change in gravity affect monsters, also. "No clients" would mean that the gravity change doesn't affect any players. "Pushables" means that pushable objects would be affected also.
I haven't found anything that states explicitly what the effect is on other players. The flags imply that all players are affected - if it's allowed in multiplayer games. I haven't found anything that says whether it's allowed in multiplayer games or not. It probably is!
You can do some experimenting with a map I put in the map vault 'Examples,' - Gravity Test.