If you are picking him up, you are scaring him. Geese don't like to be picked up. You haven't had them long, so you are still a stranger and you are frightening him. That's why he is hissing at you. He wants you to stay back and not touch him.
It doesn't mean much when a gander hisses at you. What matters is if he lunges at you or comes after you. My ganders will hiss at me when the goose is on the nest, but when I look at them, they politely turn their heads away. They also hiss at people they don't know, but don't attack. I...