Hi Mel,
You can wipe the customizer settings you’ve applied so far on Appearance > GeneratePress as shown here – https://share.getcloudapp.com/geupEppL
The content layout will be untouched but any customizer-related styles will be wiped.
If you’re pertaining to the imported Site Library template, if you edited the page added from the import, I’m afraid that’s going to be wiped if you remove the imported site template.
If this is the case, you may have to duplicate the pages to keep their layouts and then remove the imported site template and then import a new one.
Note: If in doubt, make a back-up.