In regard to Acts 13:42-44 (which SDA's often cite in an effort to make an argument for sabbath worship) Paul's work here was evangelism. Notice that these were "unbelievers" in Christ before Paul preached to them. Yes, they believed in the Jewish system, but the Bible says in Acts 14:1, that they BECAME believers proving Paul's work there was evangelism and not sabbath worship.I already addressed this.
Why would Gentiles "keep" an Israelite ordinance?
They wouldn't! They might, however, for convenience, gather with Israelites to hear the word of God when and where the Israelites gathered.
The Greeks were Jewish converts to Judaism known as proselytes. They practiced the law of Moses and kept the sabbath. The only Greeks that were in the synagogue would be these proselytes. These Greeks were not yet Christians. Acts 13:43 - "Now when the meeting of the synagogue had broken up, many of the Jews and of the God-fearing proselytes followed Paul and Barnabas, who, speaking to them, were urging them to continue in the grace of God."
Acts 14:1 - "In Iconium they entered the synagogue of the Jews together and spoke in such a manner that a large number of people believed, both of Jews and of Greeks."
Acts 17:4 - "And some of them were persuaded and joined Paul and Silas, along with a large number of the God-fearing Greeks and a number of the leading women.
Acts 18:4 - "And he was reasoning in the synagogue every Sabbath and trying to persuade Jews and Greeks."