Does Acts 28 teach that all Gentiles are saved because salvation was sent to them?
No. Acts 28 states that the salvation of God was sent to the Gentiles and that they would hear it, but it does not say all Gentiles would be saved. Hearing the message makes the world guilty and without excuse, not automatically saved.
Les explains that by Acts 28 Paul had finished his missionary journeys and was imprisoned in Rome. He called the Jewish leaders in Rome and reviewed how all the Old Testament covenant promises had been presented to Israel, but they rejected them when they crucified their Messiah. Then Acts 28 declares that the salvation of God is sent to the Gentiles, and they will hear it. But Les emphasizes that this does not mean all Gentiles are going to be saved; rather, they are going to hear it, and Scripture shows they have heard. That is why all the world stands guilty, and why Romans 1 says they are without excuse. With the advent of Paul going out, the calling into the body of Christ becomes predominantly Gentiles, with Jewish exceptions—a complete flip-flop from Jew only with some Gentile exceptions.
Be it known therefore unto you, that the salvation of God is sent unto the Gentiles, and that they will hear it.