Oh, my geese would love a creek! They'd never come out again . . .
They look just like my American Buff Goslings that are that same age. They have great personalities as adults! We love ours.
You can pull an adult feather and send it in for DNA sexing, but I don't remember what company to use for that. I think it costs about $20/feather.
Otherwise, you'll find out for sure when breeding season starts -- usually December/January - although we had fun guessing before then. Usually the males will come closer to you, are more sociable and have a higher pitched voice as adults, and carry their heads more upright. The female geese are usually smaller in size, have a lower voice, are more reserved, and carry their heads lower. Right before they start laying, their rear will fill in and look like the stern of a boat . . .