It's possible, but don't get your hopes up. There are hardware-specific quirks that are NDA'd, and you will only get a certain amount of functionality from a GSI.
If Planet get back to me, I will be broaching the subject of making these quirks and AOSP modules, open source under a GitHub organisation. This way, custom ROMs can just import these modules seamlessly.
The Cosmo is tricky, as is the Gemini, as it's A-only partitioning (Dynamic Updates). The Astro differs with the A/B scheme.
I'm also looking into making the keyboard driver (and STM32 quirks for Cosmo) into a Linux kernel module, so that mainlining the kernel is less tricky.
You may be interested in this:
https://github.com/PC-Custom-Android-PortsAll in all, it's not going to be an easy ride.