Blasphemy against the Holy Spirit would be rejection of the Gospel, because the Holy Spirit convicts the world of sin, righteousness, and judgement. In the verse you quoted Jesus said, “Whoever is not with me is against me, and whoever does not gather with me scatters.” If you aren’t with Christ, you’re condemned. If you reject the conviction of the Spirit, of sin, righteousness, and judgement, how can you be forgiven for you then have denied the Gospel.
Consider verse 33.
Matthew 12:33
King James Version
33 Either make the tree good, and his fruit good; or else make the tree corrupt, and his fruit corrupt: for the tree is known by his fruit.
Jesus is the Messiah, and they were rejecting Him; even saying His works were not from the Spirit of God but from the devil. They were calling a good tree corrupt.
Remember Jesus was speaking to Pharisees, people who did not believe in Jesus but were stuck in self righteousness. They sought a righteousness of their own, and not the one that is by faith (imputed).
To be rather blunt, there is no sin that can condemn a Christian because such a sin would supersede the blood of Christ that has granted us the remission of our sins. Scripture is plain on this point, if all sin hasn’t been paid for then Christ would have to die again and again since the foundation of the world because He is the sacrifice for sin (Hebrews 9:26). He has an eternal priesthood and His blood (that is the only means of remission) is how we are forgiven of sin. How else do the unrighteous receive forgiveness today? His blood sacrifice that still applies today, through His eternal priesthood (order of Melchizedek).
The topic itself of blasphemy against the Holy Spirit not being forgiven was directed at Pharisees, an important point. We are believers in Christ, having received the remission of our sins through His blood. We didn’t reject Christ, we received the conviction of God’s Spirit and believed. We saw the tree was good. I suppose this can be summed up in Christ’s question to others of who He is: Who do you say that I am?