To get the Gold Trophy, trade a Pokémon received from a Round 2 fight in the original Pokémon Stadium to your G/S/C game. When it gets to your G/S/C game, it will be holding a Gorgeous Box, the Gorgeous Box has a Gold Trophy inside. If you trade a Pokémon to G/S/C that was recieved from a Round 1 fight in Pokémon Stadium, then it will be holding a Normal Box, which has a Silver Trophy inside.