update: I'm going to exchange them to a 40.5. I originally purchased 39.5 which is my brixton size but it's just too short in the length like I have a bit of heel over hang. 40 would probably be perfect but it is sold out so I feel like the 40.5 should hopefully be okay.
I don't have the slingback but I have the Princetown in leather, jacquard and canvas. If the slingback is anything like the Princetown (and I believe it *is* the princetown but with the strap), depending on just how much your heel hangs over, I would be cautious going a full size up because the leather on these is ultra soft and will give as you wear them, allowing your foot to move forward. If you go a full size up you could wind up sloshing around in them. Is mytheresa the only store you can buy from? I'm not in the market for this particular shoe so I'm not up on availability (nor am I your size), but I have noticed it available from other stores... perhaps check with farfetch, matches, the gucci website... would be a shame to spend so much on a shoe and have the fit be less than ideal. FWIW I can wear either a 37 OR a 37.5 in the Princetown. I have both, and while the 37 **justtttt** fit my foot length at first when it was new, to the point I questioned it being the best size, since wearing it and the shoe relaxing the fit is also that much more generous... and that's in the jacquard, which is stiffer than the buttery leather. In the leather and canvas mine are 7.5 and I've found they've relaxed substantially.