On mainline a gate checked bag will go into checked luggage and won't be available plane side. If you hidden city ticket and have to check, you are caught.

Your bag will end up at your ticketed destination.

Moreover, trying to get access to it is a red flag for HCT with the most aggressive airline fighting this practice currently.

On edit...not sure about that last statement.AA is aggressively pursuing FF fraud, not HCT. I think UA is hot on HCT right now.