https://source.android.com/docs/setup/contribute/licenses says most of the Android userspace is Apache 2 licensed. While they can’t close source the Android branch of the kernel, they could close-source new userspace code and it would probably diverge from the last open source release quite quickly.
Realistically, that would probably be sufficent to make Android functionally closed-source, even if the GPL bits were still available.
Graphene OS drops support for devices pretty soon after Google does. It’s not a good idea to buy anything before Pixel 6 to run Graphene OS right now (see https://grapheneos.org/faq#supported-devices)
This will get better going forward since Google is supporting the Pixel phones for much longer (5 years for the Pixel 6/6a onward, 7 years for the newest devices).
https://source.android.com/docs/setup/contribute/licenses says most of the Android userspace is Apache 2 licensed. While they can’t close source the Android branch of the kernel, they could close-source new userspace code and it would probably diverge from the last open source release quite quickly.
Realistically, that would probably be sufficent to make Android functionally closed-source, even if the GPL bits were still available.