Baby likes to gag herself?

So I take care of a 1 year old little girl. I’ve noticed she’s been gagging herself, she shoves her hand in her mouth as far as she can and gags herself over and over. She usually does it when she gets bored or angry, I have no idea why she does it or how to get her to stop. I tell her no repeatedly and take her hand out of her mouth but nothing seems to stop her.