Some geese will indeed over eat and get fat.
If you're going to breed them this could cause problems
since geese gain weight internally around their reproductive organs.
For a maintenance diet they should get 1/4 lb of 12% feed for every
10 lbs of body weight and all the grass, greens or leafy hay they will eat.
Or 2% - 2.5% of their body weight is a good rule of thumb.
Breeding stock during the breeding season
can have free choice of 18 - 20% ration starting 3 - 4 weeks prior to and
during the breeding/egg laying season.
If you're not going to breed them I wouldn't worry about it but make sure
to give your geese a complete feed like a multi flock ration.
I have some tips on my website on raising waterfowl. (link under my avatar)
Good luck!