Genesis 49:19

The tribe of Gad with the rest of the tribes will, armed, pass over the streams of Arnona and subdue before them the pillars of the earth, and armed will they return into their limits with much substance and dwell in peace beyond the passage of Jardena; for so will they choose, and it shall be to them to receive their inheritance.

‘GAD G’DUD Y’GUDENU.’ Rashi, in accordance with Onkelos, explained it as follows: A troop will troop out of him, and that troop will troop back upon its footsteps, meaning it will return in its own track back to its territory in peace [and not one of them will be missing].
The correct interpretation [appears to be that the word y’gudenu here] is as in the verse: When he cometh up against the people that he invadeth (‘y’gudenu’). The verse is thus stating that a troop will always assail Gad, that he will have many wars, with enemy troops spreading out over his land, and that he will follow the enemy in his track and be victorious over him and pursue him, returning on the heel of those who shame them. Jacob thus praised the Gadites for their valor and for their victory over all those who enter into battle against them. This is similar to the blessing which Moses our teacher gave them, as it is said, Blessed be He that enlargeth Gad; he dwelleth as a lioness. Since Gad inherited a very wide and large land which was across the Jordan, bands from Ammon and Moab, his evil neighbors, would always descend upon him, claiming the land and invading it. Yet Gad lurked like a lioness over its prey, fearing not their noise, nor showing any dismay at their shouting.
This fitting interpretation I learned from the Jerusalem Talmud, where in Tractate Sotah they say: “Gad g’dud y’gudenu, a troop will come trooping upon him, but he shall troop upon it;” that is to say, bands will come to gather wealth and assail him, but he shall troop upon them and bring his troops into their land.
Perhaps the prophet Jacob was alluding to the war of Jephthah the Gileadite against the children of Ammon, as the children of Gad inherited all the cities of Gilead, and half of the land of the children of Ammon. The children of Ammon were always warring against the men of Gilead, and Jephthah scarcely passed over them and he smote them and their cities [with] a very great slaughter. This event was indeed a great miracle, and so the prophet Jacob mentioned it, even as he mentioned the matter of Samson.

גד גדוד, the troops of the entire nation of Israel will follow Gad who is the vanguard in the wars of conquest as we know from Deuteronomy 3,18 where Moses charged them with this task. On the return from battle, this tribe, instead of being in the vanguard, will be the rearguard, again the most exposed to enemy action. They will thus form a buffer for the Jewish armies at all times.

גד גדוד יגודנו, after having said that Dan fights like a snake felling horse and rider, Yaakov now contrasts Gad’s mode of operation with that of Dan by saying that the Gadites act in groups. Moses also repeats this theme in Deut. 33,20 when he said וטרף זרועו קדקוד, “he tore off arm and scalp.”

והוא יגוד עקב, when he fells horse and rider he will cut off their heels.. This is because they will try to flee from his pursuit. Compare Exodus 23,27 ונתתי את כל אויביך אליך עורף, “I will make all your enemies flee from before you.”

גד גדוד יגודנו, if a troop, battalion attacks him he will overpower that troop. This is the meaning of the last words in the verse והוא יגד עקב, in the end (עקב) he will prevail. This is a specific reference by Yaakov to what we have been told in Chronicles I 5,18-22 in which this tribe (together with Reuven and half the tribe of Menashe) is described as waging a successful war against the Hagrites. Their victory, -apparently they had been attacked,- is credited to the fact that they implored G’d to help them, and had faith that He would help them. Their trust was justified as is evident from the amount of loot they captured after that battle, Subsequent to that battle they remained undisturbed until their exile, about 150 years before the destruction of the Temple in Jerusalem.

גד גדוד יגודנו, “Troops will be called up from Gad;” if you were to question how this tribe would secure its livelihood, seeing its men of military age will be conscripted as soldiers? The tribe would obviously have to be blessed with independent wealth in order to succeed. This is why in the next verse we hear about the fertile fields of the tribe of Asher which would supply also the needs of the neighbouring tribe of Gad. The soldiers’ physical needs would be supplied by that tribe, i.e. he would supply the King’s needs, the King having to pay the soldiers.There is an interesting story related in the Talmud, tractate Menachot folio 85, about the people of Laodicea being in need of oil. They appointed an agent and instructed him: “go and purchase for us one hundred measures of oil.” The agent first came to Jerusalem and was told “go to Tyre.” He came to Tyre, and was told: “go to Gush halab.” When he came to Gush Halab, he was told to go to a certain farmer on a certain field. He went there and found the farmer breaking up the earth around his olive trees. When he asked the farmer if he had a hundred measures of oil, he was told that the farmer did have it. He was asked, however, to wait until the farmer had completed the work he was engaged in. After the farmer had finished his work, he threw down his tools and went on his way, removing any stones that were in his way. The agent thought to himself: “has this man really the quantity of oil that I wish to purchase?” He thought that the Jews had tried to make a fool of him. As soon as he reached his hometown, that man’s maidservant brought out a bowl of hot water and he washed his hands and feet. She then brought out to him a golden bowl of oil, and he dipped in it his hands and feet, thus fulfilling the verse “and let him dip his feet in oil.” (Deuteronomy 33,24, Moses’ blessing of Asher) After they had eaten and drunk, the man measured out to the agent a hundred measures of oil, and he asked the agent, if per chance he could use more than that. The agent replied in the affirmative, but added that he had not brought enough money with him for the extra amount. The man said that he would give it to him and would accompany him to his home where he could pay him. He proceeded to measure out for the agent another eighteen thousand measures of oil. It is reported that he hired every horse, mule, camel and ass in the land of Israel. When he reached his hometown, all the people came out and applauded him.” Do not applaud me,” he said; applaud this man, my companion, who measured out for me a hundred measures worth of oil, and to whom I still owe for eighteen thousand measures of oil. This illustrates the meaning of the verse in Proverbs 13,7: “some make themselves rich, yet have nought; another impoverishes himself, but has great wealth.”