After the avoidance of the Apostles' being distracted from the ministry of the word and prayer in Acts 6 through the appointment of seven godly men to deal with the fair distribution of food in the Jerusalem church, Luke declares:
And the word of God increased; and the number of the disciples multiplied greatly in Jerusalem, and a great many of the priests were obedient to the faith (Acts 6v7 - RSV).That is certainly gospel growth and the overcoming of the deployment problem in the church was crucial for enabling such growth. But critically for this question, Luke makes the word of God the subject of the verb, as he does in Act 19v20. He does not say the church grew the word.
The Apostle Paul in 2 Thessalonians 3 appeals for prayer
that the word of the Lord may speed on and triumph, as it did among you, and that we may be delivered from wicked and evil men; for not all have faith (v1-2),The prayers of the Thessalonian church are thus very important for the progress of the gospel and for the deliverance of Paul and his fellow workers from those who would hinder the proclamation of the gospel. But again the word is the subject of the verb.
This formulation surely makes sense in the light of the New Testament's teaching about the respective divine and human roles in gospel growth, as Paul explains so clearly in 2 Corinthians 4. Paul and his fellow workers preached the gospel faithfully and as they did so God brought about the conversion of human hearts, thus giving 'the light of the knowledge of the glory of God in the face of Christ' to previously spiritually blinded individuals (v5&6).
So, Christians have a role in God's sovereign plan in proclaiming the message of salvation and praying for its progress but God's unique role is the decisive one in converting individuals and thus 'growing the gospel' in the sense of more people believing it and gospel-proclaiming churches being built up. Thus, making the word of God the subject does not exclude human activity but attributes the lion's share of the growth to the right Person.
In the light of this, is it not arrogant to speak of us in our church taking actions to 'grow the gospel' in our locality? At least, even if those speaking in these terms have the right theological framework in their minds, it sounds arrogant, so is not the phrase best avoided when 'we' are the subject of the verb?