By far the most common reason for a goose (or a duck) to go off its legs is when it is carrying lots of worms. Gizzard worm is the biggest problem for geese and can be fatal unless treated early, especially where goslings are concerned.
Goslings are particularly susceptible to Gizzard Worm. Always handle goslings regularly to ensure they are not losing weight. Flubenvet is licensed to treat geese and they should be treated at the first sign of problems.
If your goose has gone off her legs, the first thing vets will usually do is worm them if there are no other obvious problems.