“May reveal to them the gospel of righteousness.” This is a petition for the catechumens during the liturgy.
Also from the first Canon of the Seventh Ecumenical Council:
And if forever the prophetic voice commands us to keep the testimonies of God, and to live in them, it is plain that they remain unwavering and rigid. For Moses, too, the beholder of God, says so in the following words: “To them there is nothing to add, and from them there is nothing to remove”. And the divine Apostle Peter, exulting in them, cries: “which things the angels would like to peep into”. And Paul says: “Though we, or an angel from heaven, should preach to you any gospel besides that which ye have received, let him be anathema”.
The link in both of these quotes is the gospel with righteousness, with keeping the testimonies of God and the way of life, that is living by them.
I understand that this means that the gospel is not merely a message that Christ died for our sins and that we must believe in Him nor is it merely that we must have a particular faith but it is that we must also live a particular life maintaining particular commandments. That is the gospel is Christ and union with Him, deification. This encompasses faith and actions that we may be united to Him in all our life and thoughts. That He can become all in all. That we may conform to His likeness to become what He is Himself. It is a gospel of righteousness, although this does not mean justification by works.
Also, what I see is important from this that St Paul’s comment that one is an anathema for preaching another gospel. From this it seems that teaching heresy is primarily: the preaching of another gospel. This does not mean merely preaching a different Creed as a statement of faith but also about preaching a different way of life because the way of life is also Christ and God. That is if one is to teach contrary to the commandments of God and Tradition of the Church in faith and practice then one is a heretic because one is teaching another Gospel. This is why the Fathers also maintain and teach the sacred Canons as “unwavering and rigid”. This is not to deny the economy of applying them but that one is a heretic to teach contrary to them because such teaching is a false gospel. Those that taught that we must be circumcised were condemned as preaching a false gospel by St Paul that is as heretics. Teaching a false commandment and action is teaching a false gospel as much as teaching falsely about the doctrine of the Trinity.