I have killed a ton of geese with my 20 gauge 870. A ton.
To be perfectly honest I usually use #2's or #4's. In 13 years of waterfowl hunting I have lost 1 goose.
I would highly recommend not using 3.5" shells (too much bang with a over-under).
You have gotten great advice thus far but this is my

concentrate on the white cheek patch
when shooting. You'll have better success. Also geese are big and appear closer than they really are.
Birds flare when people shoulder their gun too soon, this is a newbie waterfowler mistake that you should
avoid. Don't move until your ready to shoot, when you are then take your time with your shot, concentrate
on one bird in the flock. Say to yourself "I am going to kill THAT bird" and you'll look like an old pro.
Another thing. People say goose tastes bad, that is stupid. Geese are absolutely delicious. Ask for recipes if you
have success, I can really help you there.
Its awesome you got invited.