What happens if two investigators are in another world and the first out closes/seals the gate, and the second is still stuck inside? Is that individual lost in time and space? Thanks.

Yes, Lost in Time and Space, unless another gate to the same world is open elsewhere in Arkham.

Just to clarify, the investigator stuck inside doesn't get LITAS until the movement phase when they would normally leave the second half of the other world and return to Arkham.

Thanks for the answer!