The choice of Gad and Reuben to settle outside Canaan, to the east of the Jordan River, led to disastrous consequences:
1 Samuel 10:27 Nahash, king of the Ammonites, had been grievously oppressing the people of Gad and Reuben who lived east of the Jordan River. He gouged out the right eye of each of the Israelites living there, and he didn’t allow anyone to come and rescue them. In fact, of all the Israelites east of the Jordan, there wasn’t a single one whose right eye Nahash had not gouged out. But there were 7,000 men who had escaped from the Ammonites, and they had settled in Jabesh-gilead.
Nahash then moved against the people of Jabesh-gilead, threatening to gouge out their right eyes too:
1 Samuel 11:11 But before dawn the next morning, Saul arrived, having divided his army into three detachments. He launched a surprise attack against the Ammonites and slaughtered them the whole morning. The remnant of their army was so badly scattered that no two of them were left together.
Why Gad and Reuben hadn’t asked for help earlier isn’t known. Maybe no one could escape in time to bring back help. Maybe Israel was too disorganised to mount an attack until Saul had been anointed king Samuel suggests it was one of the reasons Israel wanted a king,
1 Samuel 12:12-13 But when you were afraid of Nahash, the king of Ammon, you came to me and said that you wanted a king to reign over you, even though the Lord your God was already your king. All right, here is the king you have chosen. You asked for him, and the Lord has granted your request.
The Ammonites were then no threat for about 50 years (42 years of Saul’s reign then 7 of civil war and David establishing his authority in Israel), until Nahash died. His son failed to keep faith with David, starting a war he would come to regret (see 2 Samuel 10).