Chip: I have a soft spot in my heart and soul for guns with family provenance. Those I do not sell. But, that's just me. If grandma wants/needs the $$, I would of course help her too. (I think I would give her some cash and keep my grandfather's gun, but that's just me). Anyway, to get an idea of value, check out
gunbroker.com and search for Parker 20. Recognize these are asking prices and sales often go for somewhat less. You have a plus in it being a 20 gauge and, for some, a minus because of a replacement stock. If a friend gave my grandma $2000 for the gun, I would be happy. A stranger would have to pay more
Hope this helps you a bit.
Cheers,
Jack