Can you load in to the live usb for mint and see what it looks like when you open gparted? When you reinstalled did you select erase the whole drive or another option?

Here are two videos on clean installing either ubuntu and windows just in case.

https://www.youtube.com/watch?v=bP03Y-l9NOM

https://www.youtube.com/watch?v=dOoyem2xOyc

Even if you want to keep on dual booting you’ll have to get one installed first anyways.

often times if you had backed up any important files before doing this you are gonna waste less time by just doing a clean reinstall, but you can definitely try to troubleshoot fixing the boot