Which Old Testament portion does Les say clearly states Israel's responsibility toward the Gentiles?

The Old Testament portion is Zechariah chapter 8, verse 20. It is presented as the clearest statement of what Israel was to have done with regard to the Gentiles and the rest of the world. God speaks through the prophet, saying it shall yet come to pass in the future.

Les explains that Zechariah 8:20 is as clearly put as any Old Testament portion concerning what Israel was to have done with regard to the Gentiles and the rest of the world. He notes Zechariah is just in front of Malachi, then reads chapter 8, verse 20: “Thus saith the LORD of hosts, it shall yet come to pass.” Because Israel had not yet had the promised Kingdom, Les says this was still future, even though they had experienced the glory of Solomon and David. In the surrounding context, the twelve men remain in Jerusalem, not derelict, knowing they could not go beyond Israel’s borders until Israel had her King, hoping Israel would repent so Christ could set up His Kingdom.

Zechariah 8:20

Thus saith the LORD of hosts; It shall yet come to pass, that there shall come people, and the inhabitants of many cities:

Where in the Old Testament is Israel's role toward the Gentiles most clearly described?

Zechariah 8:20 is the Old Testament passage where Israel's role toward the Gentiles is most clearly described. The prophet declares the LORD of hosts' future promise, showing what Israel was to have done regarding the Gentiles and the rest of the world, a promise still future because the Kingdom had not yet been established.