Actually, I'll just tell you. You need to manually create a triger_gravity entity.
Here's what you do:
1. Create a brush using the AAATrigger texture
2. Select the brush and click toEntity on the right
3. In the entity selection dropdown menu type in trigger_gravity
4. Unselect SmartEdit and remove all items in the Keyvalues
5. This is a two parter:
a) Put in the key style and give it a value of 32
b) The next key you need is gravity and it can have a value between 0 and 1, with 1 being full (or probly equivilant to sv_gravity 1000 in console, which is 200 over the norm). I'm using 0.2 for a skypads variation