You cannot add Premier Rewards Gold cards to a Platinum card account. You can earn standard Gold Cards, which will earn 1 point per dollar the same as the Platinum card itself.
If you add supplementary cards to a Premier Rewards Gold account, all of the cars earn the same category bonuses.
So in short, I have to apply for the PRG account, or an AMEX everyday account to get the bonuses on things like airline tickets, gas and supermarkets, correct? That is a slight let down, yet understandable.