Was God unfair to the Gentiles by only dealing with Israel in the Old Testament?
No, God was not unfair to the Gentiles. He knew they would have trampled His offer of salvation underfoot for 2,000 years. So He dealt with one nation, Israel, intending to use them to reach Gentiles. When Israel failed, He turned directly to Gentiles with salvation by grace through Christ, not Law.
Les explains that God was not being unfair, because He knew what those Gentiles would have done with His offer of salvation. They had trampled it underfoot for 2,000 years already, so God did something different: He dealt with just one little nation, Israel, and brought them to the place where He could use them to go back to the Gentiles. But even the favored nation failed. So God went to the Gentiles without the nation of Israel. This is what Paul trumpeted to the Gentile world. The Jew had those promises, rejected them, and God sent them into dispersion. He then turned to the Gentiles with a plan of salvation based not on temple worship or keeping the Law, but on the fact that when Christ died, He paid the sin debt. Because Israel was set aside, salvation came to Gentiles through the free gospel of grace, faith plus nothing. Yet God is not all through with the Jew; He is still going to come back to them.