Removing snap, reinstalling gnome

Solution 1:

The packages gnome-3-34-1804 and gnome-3-28-1804 are not the GNOME desktop environment you are using. They are the GNOME backends for GTK snap apps. You can safely remove them, if you intend to remove other snap apps. This answer explains how to.

That won't affect your GNOME desktop environment (which is installed as a .deb package through apt, and you don't need to reinstall it.)

Solution 2:

Before you uninstall GNOME, install something else. Or just uninstall gnome and reinstall gnome using https://linuxconfig.org/ubuntu-20-04-gui-installation

For core and core18, I don't know what happens if you uninstall those...