It's an interesting one. I completely agree with your first point and hope we either tell Levy to piss off or add 10m onto Gabby's price for them.
In terms of Gabby's motivation, maybe he's thinking long term and sees more of a project being put together at Spurs. Son is 30 next month and Kane is closing in on 30 and will surely need a bit more rotation as he goes on, and Gabby has time on his side to potentially become a big player for Tottenham over time. Plus, they're in the CL, while Arsenal aren't.
It's tricky though. At Arsenal, he'll seemingly get a lot more game time, work with a manager he knows and who admires him greatly, and be a much more vital part of their team straight away, whereas at Spurs he runs the risk of spending large periods on the bench once more. There's pros and cons to both. Personally I think he'd be better off going to Arsenal and I don't like the idea of us strengthening Spurs as I'm utterly sick of the sight of them.