how do i flash gapps onto aosp android 10 without custom rom, and how do i fix my ussd and call function not working?

i recently managed to flash gsi using fastboot instruction: fastboot flash system system-quack-arm-aonly-vanilla.img from https://github.com/phhusson/treble_experimentations/releases/tag/v213 they don't have twrp for my device so i had to use fastboot. i wanted to know how do i flash the gapps img. file (or how can i get gapps?) and how do i fix ussd and phone calling function which does not work? everything else works perfectly