This is the icing on the cake! But to really understand what Paul is saying we must look at the King James version. "Therefore if the uncircumcision keep the righteousness of the law, shall not his uncircumcision be counted for circumcision?" There is a difference between keeping the righteousness of the law and keeping the law's requirements.
Notice that he doesn't say the uncircumcised, or the Gentiles kept the law, but he said they have kept the righteousness of the Law. The difference is huge, because anybody can keep the righteousness of the Law through faith in Jesus Christ. But no one can keep the letter of the Law. In Romans 3 Paul says, "But now a righteousness from God, apart from law, has been made known, to which the Law and the Prophets testify. This righteousness from God comes through faith in Jesus Christ to all who believe. There is no difference, for all have sinned and fall short of the glory of God, and are justified freely by his grace through the redemption that came by Christ Jesus. " ( Romans 3:21-24 )
Do you see the difference? The Law and the Prophets testified that the righteousness of the Law came through faith in the Messiah and not through the letter of the Law and fulfilling it. Because that was impossible, and it was set up this way from the beginning.
Then Paul goes on to says in verse 27, that the uncircumcised of the flesh had indeed fulfilled the law. "And shall not uncircumcision which is by nature, if it fulfill the law, judge thee, who by the letter and circumcision dost transgress the law? ( Romans 2:27 KJV ). So Paul is saying that the Gentiles, who were not physically circumcised had fulfilled the law by their faith in Christ Jesus. Because of it, it brought judgment upon the Jew who was circumcised of the flesh and relying upon themselves.
Bringing it all together, we can conclude that we must be righteous to fulfill the Law. However, the things we do to obtain this state only brings about self-righteousness which can never accomplish or obtain these standards. Jesus on the other hand was completely sinless and because of that He fulfilled the letter of the Law. When we trust in Him as our Savior then he transfers to us, His righteousness and because of it, we fulfill the law. II Corinthians 5:21 says, "God made him who had no sin to be sin for us, so that in him we might become the righteousness of God." We transferred our sin to Jesus and He transferred His righteousness to us. Today thank God that if you believe in Jesus Christ as your personal savior then you are the righteousness of God and have fulfilled the Law according to what He has done!