First, term "gross sins" What is a gross sin according to the WTS under the Law code, and in the NT? Many are similar.
Well, reflect on the severe cutting off mandated in God’s Law to Israel. In various serious matters, willful violators were executed. (Leviticus 20:10; Numbers 15:30, 31) When that happened, others, even relatives, could no longer speak with the dead lawbreaker. (Leviticus 19:1-4; Deuteronomy 13:1-5; 17:1-7) Though loyal Israelites back then were normal humans with emotions like ours, they knew that God is just and loving and that his Law protected their moral and spiritual cleanness. So they could accept that his arrangement to cut off wrongdoers was fundamentally a good and right thing.—Job 34:10-12.
It may be hard for those relatives to apply this divine directive, even as it was not easy for Hebrew parents under the Mosaic Law to share in executing a wicked son. Still, God’s command is clear; thus we can be sure that disfellowshipping is just.—1 Corinthians 5:1, 6-8, 11; Titus 3:10, 11; 2 John 9-11; see The Watchtower, September 15, 1981, pages 26-31; April 15, 1988, pages 28-31.
Those representative members personally killed likely as many as one thousand of the disloyal Israelites. (Num. 25:3-5) A parallel to this can be found in the Christian congregation today. While it is not authorized by God to execute disloyal members who practice serious sins, it is authorized to take action against them by disfellowshiping them from the congregation if they are unrepentant. (1 Cor. 5:11-13) This is necessary to keep the congregation clean. If it failed to do this, how could it rightly claim to belong to Jehovah and Jesus Christ, who are righteous? It is obligated to uphold the righteous laws of God.