Is it correct that GRAMPS does not export shared (witnessed) events via GEDCOM? It appears that only the Primary Person’s events are exported. If so, is there any method for exporting shared events to other software? Thanks.
Correct out of the box Gramps doesn’t, but you can install the “GEDCOM Extensions” addon and when you export the GEDCOM you can then select “Exports witnesses associated with a family event” .
You could try the GedcomForGeneanet add-on. It exports witnesses, today and at event date recorded places, and more. It is designed for Geneanet genealogy web site but you may find it interesting.