Learn Turkish: Hadi, gel! - Come on, come!

In Turkish, giving a direct command to one person informally is very simple: you just use the bare root of the verb. The dictionary form is gelmek (to come), so dropping the -mek leaves you with gel (come!).

Is hadi a verb like gel?

No, hadi (sometimes also spelled haydi) is an interjection, which is a conversational word like "wow" or "hey" in English. It means "come on" or "hurry up" and doesn't take any grammatical endings.

How would I say this if I were talking to multiple people?

Hadi stays the same, but you need to add a plural ending to the verb. You would say hadi, gelin! to tell a group of people to come. You also use gelin when speaking formally to just one person.