The word συνάγων - translated to "gather" - means to "draw together, collect" and "to bring together, assemble." It is often used by fishermen - when they fish with nets.
This is the primary mission of God's representative because God wants us back, and God's representative is doing the will of God.
The Supreme Being wants us to return to Him. We were created within a loving relationship with the Supreme Being, but we were given the choice to assume that or reject it.
Those who live within the physical world have rejected that relationship in order to try to enjoy ourselves separately from Him. Because the Supreme Being unconditionally loves us, He is offering us a path to return to Him: by learning from and serving His servants.
Here also Jesus explains the law of offensiveness. While he offers that those that speak against him will be forgiven by him, those who speak against the Supreme Being will not. This is a statement coming from the intimate relationship between Jesus and the Supreme Being. The servant of God (and the servant of humankind--remember that the Greek word υἱός (huios) means either "son" or "devoted servant" or "follower") is ready to forgive us for offenses against him, but will not tolerate offenses made against the Supreme Being.
As for God, the situation is reciprocal. While the Supreme Being will forgive us for offenses against Him (as He does simply by allowing us to return to Him) He will not tolerate offenses made against His loving servant. Those that blaspheme or ridicule the humble servant of God - one whose life is dedicated to pleasing the Supreme Being - will face God’s wrath.
This loving wrath is specific to the Supreme Being's anger against those in early Israel who went against the instructions and offended His representatives such as Abraham, Jacob and Moses. Those who offended God's representative faced God's anger due to His love for His loving servant.
The safe approach is to avoid blaspheming or otherwise offending the Supreme Being or His servants.
(For a translation of Jesus' statements from the Book of Matthew without institutional sectarian influence, see the Gospels of Jesus - translated from the original Greek texts.)