How to install a 64bit gsi
You will need to recompile your entire ROM as 64 bit, then flash the GSI

GSIs will need the vendor and kernel and well everything else to be 64 bit

This is not a task for the faint heart,  but I'm sure you can find a guide somewhere. Even so, the source ROM may not have the required 64 bit libraries, so you'll have to include those. Basically the only way to bring 64 bit support is a 64 bit ROM. Not undoablle, but not easy.