WordPress 5.5 allows to update themes (and plugins) with a ZIP file:
After 11 Years, Users Will Be Able to Update Themes and Plugins via a ZIP File (in depth post at WPTavern.com)
So testing GP 3.0 just got easier and the plugin “Easy Theme and Plugin Upgrades” recommended on Beta Testing – Documentation is not necessary anymore. May be that page should be updated accordingly 😉
P.S.
I really love what I have seen in GeneratePress 3.0 so far!