They have humans at the check in counter who will handle that for you. You might need a debit card. The minimum wage union TSA screeners wouldn’t care if you had nineteen bags as long as you don’t tell them one has a bomb in it.
It really is simple with the gate check thing and if you explain (and hold firm) that that’s what you are going to do it should be just fine.
or a six-ounce tube of toothpaste...