There is no be verb, but I don't know how it becomes future tense.
Answer from a Native speaker
It should be "You are going to fit in here" but since this is a skit, they changed it to "you gon fit in just fine here." They did in fact skip "are" in this sentence. Even though this is grammatically incorrect, you do hear this type of slang used occasionally in spoken English.
1/2 STEP LEARN