GPL v3 specifically requires the vendor distributing the GPL v3 components to allow the user to change the software on the end user device. This means no more locked bootloader. We would have had choice to install custom Android distributions and thus less Google monopoly.
It was always possible to install Android alternatives, GPLv3 has nothing to do with it. I have nothing against GPLv3 of course, but this is just not true.
Remote attestation is the thing preventing the app from running on your Android alternative, whether it's GPLv3 or not does not matter. GPLv3 does not say "it's illegal to do remote attestation".