My first impression is male, my second impression is poor health the beak should be a much brighter color. To contain your birds, a fence will work much better than tying them with a rope. If you're afraid he could fly over the fence you should clip the wings.
The right diet will help tremendously. A lot of us feed our geese an all flock feed. They sell it at a lot of the farm supply stores. It comes in pellets or crumbles.
Letting your goose graze grass as much as possible will also help.
They also need constant access to water, deep enough to dunk their whole heads and clean off. Buckets work well for this. They also enjoy baths from time to time. You could fill up a baby pool or something similar for him to swim.
Tell us what feed you have available in Mexico, we know in other countries things aren't as readily available as here in the US. Beautiful goose. Congrats on getting him. Anyway you can make him/her a fence so he doesn't wander away and get eaten. Old pallets would even work. Scrap wood. Fencing? @Manuel742 Looks like a beautiful area you live in.