Try using fastboot, you can flash the ROM directly to the system partition. There are some tutorials on how to do that.

I didn’t download the image, but if it came in a .img, you can flash with fastboot, if it came in a .zip, you can flash with a recovery like twrp, what format it came in?

I wouldn’t use the pure phh GSI rom, it’s very barebones and comes with all debug options enabled.

Maybe this LineageOS miggt be better: https://github.com/MisterZtr/LineageOS_gsi/releases/tag/v2025.10.10

Or an Android 16 LineageOS (from the same repository), or other ones in this range. I have a suspicion that all these new ones are A/B compatible.

Depends on the version, if it’s android 15, use GSIs with android 15 or higher, with versions equal or higher than the vndk on your device, then it will work.

One example to the naming is in the Andy Yan’s LineageOS build:

https://sourceforge.net/projects/andyyan-gsi/files/lineage-21-pre-qpr2-light/

{arm|a64|arm64}_{a|b}{v|g}{N|S}-{signed|vndklite|personal}
|               |    |    |     |
|               |    |    |     signed: Signed with maintainer's keys
|               |    |    |     vndklite: For VNDKLite devices,
|               |    |    |               or for writeable /system on regular devices
|               |    |    |     personal: With personal mods, for reference
|               |    |    |     (secure): Superuser removed and system props spoofed
|               |    |    |               (not offered here)
|               |    |    |
|               |    |    N: No Superuser
|               |    |    S: With PHH Superuser
|               |    |    (Z): With Dynamic Superuser (not offered here)
|               |    |
|               |    v: Vanilla, i.e. no GAPPS
|               |    g: With regular GAPPS
|               |    o: With Android Go GAPPS
|               |    (f): With MicroG and FLOSS replacements of GAPPS
|               |         (not offered here)
|               |
|               a: "A-only", i.e. system-as-system (deprecated)
|               b: "AB", i.e. system-as-root
|
arm: ARM 32-bit (deprecated)
a64: ARM 32-bit with 64-bit binder
arm64: ARM 64-bit

With attention to the naming scheme, I think you can grab any that has the features you want. Oh, and microG now has a build that doesn’t need signature spoofing now, seems android doesn’t check the signatures on system apps agains the binary data, so just copying it to microG solved (from what I understood).

Depends on the GSI rom. Some have microG, some have play services, some have none, some come with debug variables enabled.

In my understanding, GSI roms (project treble) are like a shell around the main android components, like firmware, so they are generic to every device, but some features might break. Here are some ROMs.

There are some details, like vndk and vndk lite, A/B partitioning, if the ROM is signed, etc.