A different gospel is not another gospel. There is but one gospel.
But (εἰ μὴ)
Rev. only. As if he had said, “there is no other gospel, but there are some who trouble you with a different kind of teaching which they offer as a gospel.”
Some that trouble (οἱ ταράσσοντες)
The article with the participle marks these persons as characteristically troublesome - the troublers. Comp. Luk 18:9, of those who were characteristically self-righteous. For trouble in the sense of disturbing faith and unsettling principle, see Gal 5:10; Act 15:24. Not necessarily, as Lightfoot, raising seditions.