Green’s Dictionary of Slang

jell v.

also gel, jell out, jelly
[SE gel/SAmE jel, to solidify into a jelly-like substance; the image is of a jelly slowly melting; to turn into Jell-O/jelly]

(US campus) to relax, to waste time, to hang about, esp. at a soda fountain or café.

[US]V. Carter ‘University of Missouri Sl.’ in AS VI:3 205: Jelly: loiter for idle conversation in university buildings, or join friends to pass the time of day at a café or drug-store near the campus.
[US]Des Moines Register (IA) 6 Mar. 14/4: Students usually arrange for a ‘float hour,’ a vacant period [...] so that they can ‘jell’ or ‘coke and cake,’ which means loaf socially.
[US]Baker et al. CUSS 144: Jelly. Waste time, not study.
[US]Eble Campus Sl. Nov. 3: jell out – relax.
[US]Eble Sl. and Sociability 101: One set of words gently gives advice to calm down: [...] jell, and mellow out.
[US]K. Kainulainen ‘University Euphemisms in Calif. Today’ [Internet] I will consider expressions surrounding something bad or discreet as euphemism and other invented expressions as slang. Therefore ‘To gel’ meaning ‘to relax’ [...] would be slang.