GPL doesn't help you one bit in this particular situation, because "regular users" would still be using the locked-down stock Android that came with their device. So they still can't install your app.
Anyone who is already running a rooted Android or otherwise customized OS isn't affected by this, only developers who want to distribute their app to users.
AFAIK GPLv3 requires to allow user to run modified software (so essentially device must be unlockable). Android is not GPLv3, unfortunately.
Many Android devices are unlockable, you can run your own software, and yet we still have a problem. This problem exists irrespective of what you can technically do with the hardware due to the vote by corporations to favour device "security" over user freedom. A phone is useless to most people without the apps they depend on.
I literally can't unlock my device (Xiaomi) because there's no way to, despite that it's apparently the law in Germany that GPLv2 software cannot be Tivoized because "corresponding source" was interpreted to include everything you need to install modified software on the device. (AVM vs I don't remember, regarding fritz!box home routers)
I can't root or otherwise customize my OS on my device because Linux is not GPLv3.
Correct. And the reason GPLv3 exists is because TiVo did the same thing eons ago.