Romans 2:15

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)

How are Gentiles a law unto themselves? How can they even do what the law requires if they don't know what the law is? The fact of the matter is that the law is written on their hearts. Looking back to chapter one, we can see that what can be known about God is plain to them because God has made it plain to them. God is the one who has revealed it to them and so God is the one who has written the requirements of the law on their hearts. Who else can write it on their hearts? They in fact know it and were hard wired to know it by the creator. With this fact on the table, there really is no excuse. People in fact do know the law and so they can't claim ignorance.