Really need more info, and pics to pin it down. For a "range", a pristine NIB unfired Series 70 GVMT with original box and paperwork could sell to for $1000 to $1100 or so, to a discriminating buyer who knew what he was getting.
A "seasoned" but all original Series 70 GVMT., no box or paperwork and no rust might go as low as $600. Any mods, alterations, rust, etc. would take the value down from there.
Gunbroker.com is a good place to do a little research looking back 90 days to see what they've actually sold for.