The fact that they share SoC, doesn't mean kernel is compatible with all RK3188-T models. There are many things that can be different, PMIC controllers, power, battery, wifi , sound driver... The only thing to figure out for g7a is make a debug runing demesg and see what drivers and modules load at init, and trial and error, because without specific kernel code for g7a it's difficult. If you have a g5a or g7 then the ones from my github should be compatible.
You can try contact GPD and see if they can provide you kernel for g7a, it's a discontinued device so perhaps they don?t mind to share.