You can't add anything to the gospel without distorting the gospel. The false teachers have added ‘circumcision’ as a gospel requirement.

But Paul says if you add something like that to the gospel you change it. It now won't be a gospel that saves. Rather it will be gospel that leads to eternal condemnation.

Galatians 1:8-9
8But even if we or an angel from heaven should preach a gospel other than the one we preached to you, let him be eternally condemned! 9As we have already said, so now I say again: If anybody is preaching to you a gospel other than what you accepted, let him be eternally condemned!

(Note: Dominic has been painting his house. Hence the paint scraps on his hands).