Paul said this to show how strongly he felt about anyone who might proclaim a gospel other than the one he had proclaimed. If anyone would do that, that person should be cursed. Alternate translation: "But even if we or an angel from heaven were to proclaim ... let him be cursed"
# But even if we or an angel from heaven should proclaim
The word "even" shows that Paul did not believe that they or an angel from heaven would ever proclaim a different gospel. But he includes himself and angels in his statement to emphasize how wrong it would be for anyone to proclaim a different gospel.