Is showing a valid *G card sufficient to get the benefits (lounge, baggage, etc.), or does my FFN need to be in the system?

The reason I ask this is because I'd like to use benefits from one program and claim miles for another. Instead of having to go through the headache to get both FFNs into the system (since even I don't know if that's still possible), can I just roll up to check-in desk and flash my *G card and expect to get benefits? Then I can just have my other FFN imputed into the system.

Any help is greatly appreciated!

Thanks.