Can I check my bag at the gate? I'm already checked in and have my boarding pass.

Make sure you arrive late enough and they will be out of overhead space and yes.

Without knowing airlines. All have different policys it's hard to speculate

As they are about to begin the boarding process, they will ask if anyone wants to check a bag. If you are in a boarding group that is allowed a carry-on, they will check it for free at the gate.

Typically yes, just ask an airline representative.

Yes, you may check your bags at the gate

Generally yes

Yes