Battle them for a long time and make it happy using massages etc. If you go to the pokemons summary, if there is a small yellow smiley face next to its picture, then it means that it is really happy. (This usually happens with your starter pokemon, but I'm sure if you try hard enough, you'll manage with others!) :)
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you cant
no you don't give pokemon
gibble is in victory road
Pokemon Light Platinum is a rom hack of Pokemon Ruby.
From what I have heard, Regigigas is a Hoenn pokemon. You will have to trade it from Platnium. Sorry!