One of the government's contingency plans in the event of an outbreak of avian influenza is to require poultry keepers to keep their birds inside a building to prevent contact with other birds and their droppings.
Keeping geese inside is difficult. Geese are creatures for the open air. Kept inside, they will not thrive so well. Under normal circumstances, a lot of their nourishment comes from grass, so they will be much more expensive to keep. Being waterfowl, they need plenty of water to drink and normally love to bathe and play in water - all of which will create messy, damp bedding when it happens inside. The government have recognised that geese are more difficult to keep inside than other poultry, so you would therefore be allowed to extend their house by adding a secure area under, for instance, a tarpaulin with bird-proof netting all around.
Some ideas for "awnings"
Welfare - obviously there is going to be more poop and spilt water. A very thick layer of absorbent bedding - such as wood chips (not shavings) or peat is useful. Once or twice a day, pick off the poop, and replace any soaked litter as needed. Put a pile of straw in a corner for them to nose around in and nest in. Depending on the building, it may be necessary to provide artificial light.
Boredom - you could try hanging up bunches of vegetables such as cabbage, lettuce etc, to keep them occupied. You can also hang up chains for them to fiddle about with (so long as they are the closed link sort and no bits can fall off)
Feeding - You can give them free access to pellets and/ or wheat or mixed corn. You may be able to get pellets specifically for waterfowl; they are better than layers pellets. You can give them chopped fruit and vegetables. If they are not used to this, they may not be very enthusiastic to start with, so begin with small quantities till they get a liking for it. You may also be able to obtain grass meal for them to feed on, but I have no experience of this. You'll also have to give them some ordinary grit and sand so they can digest their feed properly. If you want to keep them in lay, then they'll need a supply of oyster shell grit.
Watering - You will need to put a water container in for them (a shallow one so they don't up-end in it and drown). If it is possible, place the container on a grid, or on a part of the floor with no litter, so they can't make a mud pie. Place a rock or two in the container so they can't knock it over. If the birds have access to a secure outdoor area as described above, then place the water in this area.
Fighting - If there is more than one gander in the house and they do not get on well together, they may fight when confined. If is it possible, put up partitions in your building so that there are not two males together. Alternatively, you could put all the ganders together and all the females together. (A group of ganders with no females don't tend to fight much.)
Disease Security - If your building is not secure against small birds, then put close mesh netting over the openings. Keep visitors, including delivery vehicles, and animals and vermin away and disinfect your hands etc. Keep special boots and other clothing for the secure area. Avoid contact with other poultry or birds, poultry keepers and poultry equipment from elsewhere. Ensure your feed and litter come from safe sources and keep your feed store and straw and litter store bird-proof and rodent proof. Ensure the water has not been contaminated by wild birds.