Do T-shirt designs fall under copyright protection?

Let’s say GAP has 25 T-shirt designs a year. Are these all registered Trademarks? The answer is no from what I see. So which intellectual property right reserves any given t-shirt design to GAP?

I think it must be copyright. I want to make t’s for my little biz so I was just wondering about this so I can make sure that I am protected. I have no Trademark although I will be applying for it at the same time that the t’s are on sale. I know that protects my logo as long as I proceed to using it. The main thing I am wondering is if T-shirt designs are just copyright protected by their very nature, once they are printed.