There are 2 areas of Garand stock fit which are most liable to wood compression leading to a loose fitting stock. One is with the 2 sides on the bottom of the stock where the trigger housing seats against the stock. The other are the interior surfaces of the stock where the rear "legs" of the receiver contact the stock. The military fix for this was to cut out the wood in these areas and to glue in wooden blocks which were then re-shaped to restore a tight fit. In the later days of Garand use the US military used glass bedding to restore the required amount of tension for the trigger housing.
To restore a tight fit behind the rear legs of the receiver which will prevent fore/aft movement of the receiver in the stock, you can cut thin wooden shims and glue them to the 2 stock bearing areas until tightness is restored. For the trigger housing area you can also glue wooden shims of sufficient thickness to the bearing areas until proper vertical tension is restored. There should be enough tension here to allow you to feel it as the rear of the trigger guard passes the trigger as the guard is swung upwards to lock into the receiver. A temporary fix to restore tension here, short of glued in shims, is to cut strips of those magnetic advertising stickers and place them against either side of the trigger housing before installation.
One big reason for a loose fitting trigger guard is because of worn locking lugs on the trigger guard. These should be fully round. They do tend to wear flat from prolonged use. The fix here is simply to replace a trigger guard having worn lugs with another one that has full, unworn lugs. These lugs should be greased to prevent undue friction and wear when the trigger guard is closed and locked up into the receiver.
The glue in shim approach behind the rear legs of the receiver as well as the use of magnetic shims between the trigger housing and the stock surfaces are quite unobtrusive. A purist would always say "do nothing to the stock", but this would be a minimalist approach. Under no circumstances should the top surface of the stock be sanded or shimmed to restore vertical tension. The correct solution is always from the bottom.