You can get a replacement glass (glass only!) for less than $10 at some chinese online shops, but this requires a lot of professional tooling and skills to do the repair. Alternatively there are also some chinese shops offering display units for $150 - $200. The repair is much easier to do but you will loose NFC functionality and therefore Apple Pay since this is bound to the original display unit.
I just tried putting a display of an AW3 GPS onto an AW3 LTE. I even tried moving the original NFC module of the LTE version onto the GPS’s display and ended up with pairing issues. Was able to solve the pairing issues by using the display + NFC module from GPS version but had to sacrifice support for Apple Pay. There is already a discussion about this topic here: Why won’t my watch pair after screen replacement?
"Or can i buy a digitizer for AW2 and use it to replace the one broken on the AW3 GPS version" This should work fine. There isn’t any difference in the digitizer from AW2 vs AW3 - also between GPS and LTE.