n3t_admin
Apologies I could have been more clear: I use a custom rom (voltageOS) that is capable of implementing sandboxed gapps from grapheneOS.
Despite obtaining a strong integrity, gpay refuses to work with sandboxed gapps (but works fine when you flash gapps). I suspected this had something to do with the absense of GSF, but I can not be sure.
Either way this post has gotten off-topic. I finally managed to install GSF (through a magisk module 'Universal app installer') but this has not fixed my issues.