3 We always thank God, the Father of our Lord Jesus Christ, when we pray for you, 4 since we heard of your faith in Christ Jesus and of the love that you have for all the saints, 5 because of the hope laid up for you in heaven. Of this you have heard before in the word of the truth, the gospel, 6 which has come to you, as indeed in the whole world it is bearing fruit and growing—as it also does among you, since the day you heard it and understood the grace of God in truth,
It's useful (and hard!) to think about whetther the gospel is growing and bearing fruit in our lives. Am I growing and maturing as a Christian? Can I see more of the fruit of the spirit - love, joy, peace, patience, kindness, goodness, faithfulness, gentleness, self-control - in my life? The natural conclusion is to want to try harder - "I just need to try to be more patient" etc. But that is not how any sort of fruit is produced. An apple tree doesn't have to try to produce apples. As long as it's in the right conditions (soil, light, water), it will produce apples naturally.
It's the same idea for us. As long as we are in the right conditions, the fruit will appear naturally and, according to Paul in this passage, it's all about hearing and understanding the gospel. A lack of growth and fruit in our lives might indicate that we have forgotten, are not applying, or have never fully understood the gospel. We need to be reading about who God is and what He's done in the Bible,talking about it with other people, praying and asking God to reveal more of Himself to us. The gospel isn't something that we hear and understand when we become a Christian and then move on from as we mature in our faith, understanding the gospel is how we mature in our faith.