Can IFR GPS installations be done as minor alterations also under
AC20-138a? If not, does 138a at least simplify the 337 process?


No, and that is due primarily to the flight manual supplement that is required.
AC20-138A does make it pretty simple however.

So, an IFR GPS installation is done in two steps, first the VFR installation
(log book entry only) then, second, the 337 is filed with the flight manual
supplement and flight test information.