Mike Pence may be a wonderful human being and a great family man, but exactly no one would miss him on the ticket.
In this scenario, Trump could see Gabbard as a way to try to break out of his box with hard-core supporters and get to the sort of Ron Paul-ish Democrats on the far left who are maybe reluctant Dem voters or maybe the kinds of people who don’t normally show up to vote. Maybe Gabbard even helps him get a point or two back with women.
The point is: If the race is essentially static and locked in to where it’s been since the spring of 2020, Trump will need to destabilize it in the hopes that some chaos allows him to improve his position. Trump-Tulsi would destabilize the race.