Kind of a theoretical question. If I have a device already set up running …say full Ubuntu desktop focal on RPI and I install snapcraft. Can I just snap install … say rpi-gadget and will it be converted to ubuntu core? Or is to simple an assumption?
no, that will not work, a classic install (Ubuntu Desktop) can not be turned into a Core install at all … Core uses snaps for everything, i.e. the kernel, the rootfs as well as the gadget (bootloader and device specific files) all come as snaps in an Ubuntu Core install … the boot process is very different from a Classic boot, there are only very few limited directories writable on a core system and there is no dpkg/apt support at all …
While there is an ability to build a classic image with a gadget snap to allow you to define certain interfaces only gadgets can provide, this must happen at image build time of your iso.