Now if, by some magic, I was able to synthesize a Gaisler GRlib core (vhdl sources), is there some guidance on if GPL third-party code would be allowed on the shuttle? (see https://www.gaisler.com/index.php/information/ordering/orderlicensing?task=view&id=177 )

Jeff DiCorpo (@jeffdi) was involved in setting up the license checking script and negotiating with Tim Ansell which licenses will be accepted, so he could probably tell you.