It's along the same lines of "If you can't take it, don't dish it out."
It means that, whatever you do to someone else, you should be willing to have it happen to you.
It's usually used when you do something bad to someone and then something bad happens to you. If you complain that something bad happened to you, they might say, "What's good for the goose is good for the gander," meaning that you got the same thing that you gave.
By the way, a "gander" is a male goose, not a group of geese.
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