Bridges can be painted on whatever gel by default.

Check also if your level has a poor lighting in that area where the gel is to be used, this could be related too.

Thinking it better, its maybe a compile issue. Im using fast velocity in all the Default mode. Its because it takes CENTURIES to compile in Full mode. I will try it in Full mode, though.
And here it is the map:

Imagine that a skybox is like a fake window to the outside, ok? so the way to go about it is actually texturing on the toosskybox texture only some pieces here and there to fake the sensation that you are surrounded by that sky; but NOT to actually surrounding the whole map with this, understand? Why? because you are forcing you entire map to be rendered at the same time.

Example: your map begins with a tube ride downwards and you would like the player to watch around and feel like surrounded by this skybox, ok, then make some surrounding brushes around the vactubes only textured on toolsskybox. Then go one step further and close the area where the elevator lands texturing the surrounding brushes with toolsskybox, but close it. And so on... The only problem you will meet is that sometimes you'll be able to watch through the skybox some of the rooms and stuffs behind these toolsskybox textured brushes, but that can be corrected by using areportals and keep closed those further areas so they are not rendered while the player is looking "at the sky"...

I hope you get the picture