At its core, Project Treble is designed to accelerate the update cycle of Android devices by separating the Android OS framework from the hardware-specific software, known as the vendor implementation.
The only thing "unusual" with the A6 is that it's a 64bits CPU running 32bits OS, with 64bits binder. But that's standard in Android Pie. So if you build pure AOSP GSI, you just use aosp_arm target. For my GSI, that would be treble_a64_x