We have had this discussion a few times before on this site. Some of us would say that Calvinism is the gospel and thus, all non-Calvinists believe "another gospel."

The gospel is God's message about Jesus, not the "technical manual" on how and when His work was and is accomplished.

But that's not what our friends mean when they say "Calvinism is the Gospel." They are describing the difference between monergism and synergism - whether election and saving faith and conversion are some sort of collaboration between God and men as "equal partners" who come to an agreement, or whether salvation is solely and entirely God's merciful act upon those He has chosen, who are helpless to agree to anything, held heart and soul in bondage.

So the simpler question might be: Can a person who believes himself to be an "equal partner" with God in deciding his own fate truly be a follower of Jesus? Is such a person truly converted who can still deny the absolute sovereignty of God and relying solely upon His mercy and grace? Can a person be saved relying on his own merit (at least the merit of having had the good sense to choose Christ) as much as he does on God's grace?

Nope.

And in that sense, to be sure, Calvinism is what one must eventually come to in order for his faith to be in Christ alone, by grace alone.