You can only get regigigas only after beating the elite 4 and getting the national dex. It will be in snowpoint temple and you have to have registeel,regirock and regice in your party.
No, you can not find regigigas in Pokemon Heartgld without using an Action Replay.
You can't without an action replay
You Can Transfer Your Crappy Pokemon To Platinum From The Hoenn Region Games, But The Most Accurate Way To Do It Is Of Course Use The Movie RegigIgas Action (Sorry If Action Replay Isn't Your Style)
where to find a azure flute without action replay and cheating in pokemon platinum
You can't find all the regis (without the use of an event Regigigas) in Platinum but you can find Regigigas and you need Regirock, Regice and Registeel to enter the Snowpoint Temple and if you have action replay you can try catching the 3 regis too.
only with action replay!!! there is no way without action replay
Regigigas sky form is not real
In platinum you can catch Giritina, Palkia , Dialga, Heatran, Moltres, Arcticuno, Zapdos, Cressilia, Uxie, Mesprit, Azelf, and Regigigas. With the action replay you can also get Shaymin, Darkrai, and Arceus.
you use an action replay
You can't.
Mystery Gift.
You either have to migrate Pokemon or receive them in trades