Gel creating/modding
Anyone know where the codes for the gel are?
In the EXE file, where you can't get at them. 
But you can use the "Portal 1 method" if you feel ambitious, you cover the floors and walls with a grid of little triggers to detect specific prop_paint_bombs and spawn something when they're detected. This means you can't use gel streams, and it really limits how you can build the map, but it's pretty much the only way.
Of course, if you don't want the gel to do different things depending on the angle of the wall (unlike all the original ones), you can do what Reepblue did here--use a fancy point_template for your gel bomb, with OnExploded spawning something else. That's how I made antigravity gel a while back. It spawned a spherical trigger_vphysics_motion around where it landed that reversed gravity within it.
And if you want another gel color to use, buffer overflow gel 6 is a fairly safe one. Carl's using it for Adhesion Gel and 1 for Reflection Gel, since 6 has purple splatters and printable texture names (materials/paintblob for the color of the blobs, CPropPaintBomb for the particle effect when it splatters).