Grove v0.308 (and beyond) will be saved in a file simply named GROVE. We're trying this out to make other OS and people using the itch.io app's lives easier.
Anyway, it should definitely be there on your PC, right in the spot you selected to save it. Worse comes to worse, use your PC's search function and look for the GROVE folder.
There's a save file transfer guide included in the download that'll walk you through the instructions in doing so. You should be able to manage it on your own, it's not much more than copy-pasting.