Hi there,
If I install a GeneratePress theme to replace the current theme (in a staging environment), then the entire current database will be wiped out and replaced by GP database well, correct?
No.
GP writes its customization settings to the Options table in your database.
And i assume this is the case for your current theme.
So switching theme will not overwrite your current themes settings. Which means you should be able to switch between themes and see each of their templates and settings.