When we speak of a group or a team coming together to form a cohesive whole or when we write about an idea becoming a concrete plan of action, we usually use the spelling jell for the verb. However, gel can also mean something has taken definite form.
I like to reserve gel, though, for congeled, as when a gelatin sets. The dessert gelled; the plans jelled.
Follow this link to a longer treatment of gel vs. jell.