If I close the gate will it kick visitors out?

If anyone is on your island, yes, it will force them to leave. I've asked a host to do this after a trade when I got lost on their island. I don't know if the visitors keep anything they took, because I was asking a Villager to come to my island rather than picking up things.

Gnac I close the gate all the time when i have visitors & it never kicks ppl off. I do it so other ppl that aren't invited don't turn up

I might be getting mixed up with closing the session, but if you close the gate, then surely visitors wouldn't be able to leave?

Might be ;-). Nope visitors already on your island can leave whenever :-).

Closing gate doesn’t kick visitors out and they can leave whenever. Ending session kicks visitors out.