Pedro, a Portuguese biologist who is doing bird research in the EVOA Visitor Centre area told me that Greylag Goose are not common at all in this area.
We were both sitting in a bird hide when these two Greylag Geese flew in to join the large group of Northern Shovelers, Green-winged Teal, Gadwall and Northern Pintails.

EVOA Visitor Centre area, Tagus Estuary Nature Reserve, Portugal, 12 October 2021