Hi there,
switching themes poses no issue, you will just lose certain functionality of the theme, such as the customizer settings you applied or the GP Elements you may have added.
for GenerateBlocks you could keep that installed regardless of the theme.
If that is removed though then it will leave the HTML behind and WP will give you the option to convert that content in the editor.
But what it won’t do in any instance is leave a load of shortcodes to clean up 🙂