Can a believer today in the Israel gospel legitimately embrace that gospel? 
The apostle Paul says "yes," as there is a remnant of faithful Israel believers in the current era (Romans 11:5).

But the Concordant Publishing concern has historically said “no.“ They think anyone who names the name of Jesus today, even if it is Jesus as Israel's Messiah with no accompanying grasp of the foundational elements of Paul’s gospel, are automatically in the body of Christ. They are “ignorant Paul believers.“

This is a rare instance when I completely disagree with the teachers of the Concordant Publishing Concern. Their belief not only clashes against Paul’s assertion in Romans, chapter 11, but disrespects and delegitimizes the hundreds of millions if not billions of people who know nothing of Paul but who gravitate toward all things Israel. Are not these those to whom God will bring the new covenant (Ezekiel 36) during the tribulation, at last apprising them of the truth concerning their calling?

The alternative is that there will be billions of people snatched away when Christ returns in the air, with not one faithful Israelite left upon the Earth. Is it God's intention to start from scratch with Israel after the snatching away? Our apostle says, "No way!" And so do I.