What would have happened if Kate had married Garth?

Humphrey would've gone on to becoming a lone wolf for the rest of his life. To him probably, he has no happy life without Kate.

Humphrey would have left Jasper. (In the movie, when he was about to left Jasper, he wanted to enter into the train)
Lilly would be heartbroken and maybe she would also leave Jasper.
Kate and Garth wouldnt be happy together, because they realized, they were in love with Omegas and if they would confess it, the packs would fight.

If Kate would have married Garth, Humphrey would have left Jasper and gone down a darker path that could easily spell disaster or death for him. Everyone would be depressed, especially Kate, who misses Humphrey the most.

Another possibility is, that Kate wants to be Humphrey's mate so bad that she runs away from Jasper and becomes a lone wolf, just to be with him, and Lilly and Garth do the same and stick with Humphrey and Kate.

Humphrey would love Lilly. The packs would have an uneasy alliance.

actually, i believe humphrey wouldnt be in jasper, cause around the end, he tries to board a train to leave

He would Have left Jasper, and became a Lone Wolf for the Rest of his Life... (Possible) But maybe after a few years, he loved Kate so much that he Commit Suicide...
(Suicidal Thoughts)