If the fuel gauge is right over off the dial to Full then yes, the ground isn't there. Usually, the wire that mounts on the underside of the rear panel behind tank is loose, broken, or just needs a clean where it mounts to bare metal. That's the easiest fix and first to check.
The other things to check are the ground cables from battery to radiator support, headlight harness to each end of support, and ground on dash, and anything else that has perhaps been apart or given trouble.