'But even if we or an angel from heaven should preach to you a gospel contrary to the one we preached to you, let him be accursed.'
The gospel is the most precious message in existence because it is guarded by the greatest consequence. It is is more wonderful than anyone has ever imagined, prayed, or thought. In Paul's mind getting the gospel wrong- preaching a false gospel- tarnishes its treasure, reducing it to insignificance in the eyes of others. The consequence of doing this is anathema.
'The kingdom of heaven is like treasure hidden in a field, which a man found and covered up. Then in his joy he goes and sells all that he has and buys that field.'