Weapon_PaintGun
DLC2 seems to have added some significant things to the portal2.fgd file, such as working futbol spawners, some sort of point nugget (?), and...the Paint Gun. Has anyone experimented with this? There also is an option in the Co-op spawner to give Blue a normal portalgun and Orange a paintgun, although I don't know if it works.
paintgun isn't an entity.
It was removed in early development when Valve wanted players to spread gel with portals instead of a gun.
well thats interesting as I actually had some conversations with Valve employees asking if they would polish those entities, they said that they didn't have the time or need... but perhaps they got bored
the weapon_paintgun was the entity they used while developing paint in the source engine
I know. But does it work at all? It wasn't in the FGD before, and now it is, so they must have done something.
Or maybe they're just adding in the !FGD stuff from before, and I'm completely mistaken. That's more likely. 
well the paint gun was added to the fgd in DLC1 and futbol entities were in since the beginning
After a bit of experimentation it seems that the paintgun entity is completely broken. It's a shame.
you could edit the weapon_cubemap (it is unused) to have a new model, then add a poin_clientcommand to "give weapon_cubemap" onmapspawn. then a game_ui to detect portal fires.
orange fire - speed gel
blue fire - bounce gel
Aperture Science: We do our science asbestos we can!