Gel creating/modding
Does anyone know if it is possible to create a gel, because I have some ideas for some types of gels, but I'm not really sure how to 'create' one. Might there be a tutorial out there for it?
Sadly you're going to need someone who knows how to code things if you want to do that, it can't be done in Hammer.
ReepBlue made an interesting Electric gel:
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Fuck knows how he did it though.
I will go ask him then. I got some neat puzzle ideas, like gels that cause gels to bounce, and one to make portals bounce
To my knowledge, it is not possible to "mod" Portal 2 at this time.
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I distinctly remember me, Omnicoder and whoever else sending e-mails to Valve a while back to specifically ask for a way to modify gel properties through a text file, or something similar. I hope this bears fruit, so to speak
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It looks like a prop_paint_bomb is spawned & it's not a proper custom gel type using info_paint_sprayer.
Then I guess there are triggers on the walls (trigger_multiple with filter_activator_name/class) that spawn a template (point_template with env_entity_maker) at the !activator position (onStartTouch;entitymakername;forceSpawnAtEntityOrigin;!activator), & this spawns trigger_hurts, particle fx etc wherever the paint bomb splats.
It's probably possible to use the same technique on info_paint_sprayer gel, but would be expensive & messy with the huge amount of ents being template spawned.
I think that's the limit of i/o in changing paint properties. I wonder if vscript could do more?
Valve originally planned on adding a gel that enables you to walk on any surface the gel is on. Sadly they removed it due to motion sickness in test players.
I say: **** those wimps, I want my glue gel 
I've never experienced motion sickness in any game, including those with little amounts of reference points, like Mirror's Edge... I have never understood why people get sick playing games!
But yeah, sticky gel would, as it changes perspective and if you do that repeatedly enough times at a certain frequency, it'll cause everyone but the most stout-stomached to become sick. It's science.
You can still recreate an illusion of walking on walls using a linked_portal_door.