Usually in the tournaments god cards arent allowed from a few reasons:
1. in jtp god cards arent real only fake gods where actually are only cards with atk modified and image replace
2. they dont have the effect of the real god cards
3. not everyone has the god cards version and usually because of that weird bugs appear
But in normal duels if your opponent agree you can use god cards or if you want you can do a tournament with god cards but make sure that everyone is using the same version of the game to avoid weird bugs