On what basis do Protestants extend the New Covenant to include gentiles?

Given "sola scriptura" and the explicit assertions in scripture regarding the participants in the covenant ("the houses of Israel and Judah") how do Protestants justify teaching it to be for gentile Protestants? >Jer 31:31 KJV - 31 Behold, the days come, saith the LORD, that I will make a new covenant **with the house of Israel, and with the house of Judah**: > >Heb 8:8 KJV - 8 For finding fault with them, he saith, Behold, the days come, saith the Lord, when I will make a new covenant **with the house of Israel and with the house of Judah**: Isn't this overreach that is contrary to sola scriptura? Update for clarification ------------------------ I'm not concerned that the *gospel* only applies to Jews (gentiles do have "access by faith into the grace") but rather the *covenants*: >Rom 9:4 ASV - 4 who are **Israelites**; whose is the adoption, and the glory, **and the covenants, and the giving of the law**, and the service of God, and the promises; I'm seeking maybe a "theodicy" about gentiles and the new covenant from a representative "Reformer" and why, since they are all about "sola fida" and "sola scriptura" do they commandeer the explicitly Jewish new covenant as well?