EDIT: All kinds of help needed, one question at a time
Good plan. Keep restarting until complete. Make sure to upload all failed attempts to the WIP section so your final map will look good by comparison.
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"Oh, in case you got covered in that repulsion gel, here's some advice the lab boys gave me: DO NOT get covered in the repulsion gel."
If your map has problems compiling you might want to ask someone to look at your vmf.
Once again:
1) Check if gel is enabled in your map
2) Make sure all your compile options are on normal. FAST compiles DO NOT display gel in game.
3) Make sure you have at least one light source.
4) Make sure your map has no leaks.
if your map has leaks and light the light won't be rendered properly and the gel won't be displayed 
~Lp
I have gotten some missing props in my bsp when I used prop statics. They would not show up until I made them prop dynamics. I found that this was one of those yellow errors, so I would suggest checking into yellow errors too.
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Always go to the info tab if you want to add models, then check which kind of model it can be.
Statics cannot move, are the least expensive,
Dynamic can move but are more expensive
Some yellow errors can indicate problems, so it is always a good idea to check your log indeed =D
Paint doesn't appear on certain textures, try using different textures in your map.
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Paint doesn't appear on certain textures, try using different textures in your map.
is there a list?
probably not, just choose some different textures. most work. grates obviously dont. some random ones dont. try black_wall_002_a, theres one of the top of my head that definetly works.
I think in terms of boolean variables. Generally, it makes things easier.
red text, always with the damn red text,
and just why the hell is it that the button setup as indicated for drop devices always comes out red and broken, yet still functions? now im also getting errors on my brushes and stuff being too big, and memory overflow? are you kidding me? there were gigantic rooms in portal that i bet would have had the same damn info on overflow, my map isn't nearly as big as the long ass drop to the basement with potatOS.
F***!!!!!!!!!!!!!!
The inputs to instances comes up as broken. Always. Get used to it.
I think in terms of boolean variables. Generally, it makes things easier.
I've found that the most common cause of red text is leaking. A leak is if your map isn't completely closed in by world brushes. You'll see ***LEAKED*** somewhere in your compile log and the map will not compile. You can go to Map -> Load pointfile to see if there is a leak. A red line will appear linking the outside of the map to an entity inside the map. Fix wherever it's getting in and recompile.
If it's not a leak, Hammer will ask you to navigate to a pointfile. Close out of this window, compile, and post the compile log. Red text usually means the map did not compile.
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Four Corners[/spoiler]