Stick a modified choke in it, buy some 3" BBs or hevi shot, and go hunting. This gun is a 3.5"? You might consider 3.5" Ts if so. I hope they're decoying well when you go, though, LOL, if you're using BB. Ts and hevi shot will give you close to 20 yards more thump in my experience. The 20" barrel will work just fine. I have a 24" on my goose gun. Well, it's really a turkey gun, but I couldn't get the 32" and I prefer the shorter barrel, anyway. Mines a 10 gauge and I shoot T shot out of it with a modified choke for 90% plus patterns in a 30" 40 yard target. Generally, a modified lead choke will work bets for a full choke pattern in steel, but it always helps to pattern the load if you have time.
My 12s all have 28" barrels, but my 20 SxS is a 20" coach gun and has no problem reaching out 40 yards with good patterns on dove. There's not a lot to lose by using a 20" gun if you can swing it smoothly. That's the catch, the gun will balance a little closer to you and might be a little whippy on steady flying geese. You gotta resist the urge to point shoot and follow through with your lead. I sometimes have to concentrate pretty hard on that if I'm rusty.