I'm sorry, but Mamoswine didn't exist when Gold version came out so it is impossible to get unless there is some hack that makes Gen 4 Pokemon appear in the game.
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The code for Mamoswine is POKEMONDOTCOM.
A Mamoswine evolves from a Piloswine which evolves from a Swinub.
If you can trade Pokemon from crystal version to gold version my guess is yes
you can't get mamoswine in Pokemon leaf green because it doesn't exist in leaf green
Mamoswine doesn't evolve either way because it doesn't exist in Pokemon LeafGreen. It evolves from Piloswine who evolves from Swinub, but Piloswine can't evolve into Mamoswine in LeafGreen.