Can gel be teleported/ trigger a trigger_proximity?
Fake it.
Place a trigger_multiple under the gel sprayer that detects only physics objects. Parent a info_sprayer onto the 2nd cube that activates when the trigger is triggered.
Well, that's what I would do.
Just throwing out a random idea here, but the cubes have an "OnPainted" event. You could use that to detect when one cube receives paint, and then turn on the paint sprayer attached to the other cube.
To detect when the 'input' cube stops getting painted you'll probably need to clean it periodically with a trigger_paint_cleanser. (I don't know if this is strictly necessary; you'll need to experiment with the OnPainted output and how it behaves.)
Other Portal 2 maps: Medusa Glare
Portal 1 maps: Try Anything Twice | Manic Mechanic
I think (might be imagining stuff here) that I saw Valve 'detect' a blue paint bomb when I decompiled one of the underground maps. [It was the one containing this puzzle here]. I'll look up the details when I get the chance
Co-operative: Lightspeed Upstairs Downstairs