So I bought this on gumroad before everything vanished and just wanna grab a up to date version of it and wondering if I have to re buy it to do so
The gumroad version and itch.io are the same version. You can download files from gumroad still even if the product has been taken down. There's no good way for me to copy ownership from gumroad to itch.io so you will have to buy it again if you want it on itch.io.
I consider it complete and don't plan to update it in future either, so you don't need to rebuy.