can gel be detected?
i have seen another thread, but it was for a whole nother problem. i need a way to find out if a small surface, (portal-size) has been hit or painted on with white, or any paint. im doing this as a way to know if someone has been trapped in the map. i am about to 
because this is getting to make me 
please help.
The only kind of gel that can be detected is "prop_paint_bomb" because it's a physical entity. You can detect it like any other entity with a trigger_*.
If you use the sprayer I have to disappoint you, because you can't detect that. You could use water as a workaround in that area to get rid of gel though.
welp, point_template 's and env_entity_maker 's it is then!
Gel probably is detectable somehow, otherwise it won't be affected by physical forces and objects such as point_push, tractor beams or collide with lightbridges etc.
How you can actually detect it is beyond me for the moment.
is a prop paint bomb a pushable, or a physics object?
also, what logic entity can i use that loops, so i can trigger something constantly every .5 seconds?
Prop physics. Also, you can create a logic_relay triggered fireuser1 on map spawn.
Onuser1 trigger whatever you want
Oneuser1 !self fireuser1 0:05
Aperture Science: We do our science asbestos we can!
Ha! I'm working on the same thing right now. Here is my workaround for detecting gel on a surface.
The only thing I know that can detect gel is prop_weighted_cube. So if you want to detect gel on the certain surface, create there a cube and place it so that it only protrude 1 unit over the surface. Copy it and place the new one a bit over the floor and give it a name. Turn the first one (that in the place) into point_teleport with entity to teleport - your named cube. In logic_auto set outputs OnMapSpawn >> point_teleport name here >> teleport, OnMapSpawn >> cube name here >> DisableMotion >> delay 0.01. If you just place the cube into brush it will pop up on map spawn.
Now we should make this cube invisible. Open the cube's properties, turn off smart edit and add new parameter with key rendermode and value 10. This makes the cube to be not rendered. Now you can go to outputs tab and add new output OnPainted >> whatever you want.
Hope this will help.
EDIT. Oh, I forgot one thing. In cube's outputs add OnPainted >> !self >> kill >> delay 0.5 to remove the cube after it's done its mission.
That's very clever and I'll definitely use that in some future maps 
.
A logic_timer.