Romans 2:15b

since they show that the requirements of the law are written on their hearts, their consciences also bearing witness, and their thoughts now accusing, now even defending them.)

(v15, NIV)

Gentiles who do not have the law are a law unto themselves. They show this by doing the law. How can they do the law when they weren't even given the law? They in fact show that the requirements of the law are written on their hearts. The creator of the universe created each and every one of us and wrote on our hearts the requirements of the law.

What does it mean to bear witness? This is the viewing and confirming of something in particular. I think of a trial where a witness stands and gives a testimony of something that happened that they saw and claim to have happened. Their conscious bears witness to the fact that the law is written on their hearts. How does this even work? The conscience either accuses or defends a person. Your conscience may cause you to feel guilt, accusing you of doing something wrong. Alternatively, your conscience may defend your actions, trying to justify them. Either way, this is your conscience bearing witness to the fact that you have the requirements of the law written on your heart. If you didn't have requirements of the law, you wouldn't be able to accuse or excuse yourself. You don't have an idea of what's right and wrong since you haven't been given the law. But somehow, Gentiles who weren't given the law like Jews were seem to be able to accuse or excuse themselves, showing that they have the requirements of the law written on their hearts.