To run the exporter, select "Gedcom Extensions". Select the relevant options, and the data should be exported to a file with the extension ".ged2".
Currently, there is not an associated .ged2 importer in Gramps, so the current export is one-way only (out of Gramps). It would be easy for someone to also write an associated importer to handle the extensions to bring them into Gramps as well.
= Import Gedcom Extensions =