John Gill Commentary 2 Samuel 5

Then came all the tribes of Israel to David unto Hebron
All the rest of the tribes, save the tribe of Judah, who had made him king over them in Hebron seven years ago. These were ambassadors sent in the name of the several tribes to him, quickly after the deaths of Abner and Ishbosheth; from having any hand in which David had sufficiently cleared himself, and which had tended to reconcile the minds of the people of Israel to him:

and spoke, saying, we [are] your bone and your flesh ;
for though he was of the tribe of Judah, yet as all the tribes sprung from one man, they were all one bone, flesh, and blood; all nearly related to each other, all of the same general family of which David was; and so, according to their law, a fit person to be their king, (Deuteronomy 16:17–18:13) ; and from whom they might expect clemency and tenderness, being so near akin to them.

Also in time past, when Saul was king over us
Even over all the tribes of Israel:

you were he that led out and brought in Israel ;
that led out the armies of Israel against their enemies, fought their battles for them, obtained victories, and brought the troops under his command home in safety; and the remembrance of these valiant acts of his, which then endeared him to the people, was now another reason for their choosing him king: and another follows, the chiefest of them all:

and the Lord said to you ;
when anointed by Samuel; for though what follows is not recorded in so many words, yet the sense of it is expressed in the anointing him to be king, whose office, as such, lay in doing the following things:

you shall feed my people Israel ;
as a shepherd feeds his flock; hence kings were frequently called shepherds, and David particularly, in which he was an eminent type of Christ, see (Psalms 78:71Psalms 78:72) (Ezekiel 34:23Ezekiel 34:24) (Ezekiel 37:24) ;

and you shall be a captain over Israel ;
the Targum is, "and you shall be king over Israel;" which gives the true sense of the tribes, and which was the chief and prevailing reason with them to make him their king; and which they, at least many of them, would have done before, even immediately upon the death of Saul, but that they were persuaded by Abner to yield obedience to Ishbosheth he set up.

So all the elders of Israel came to the king to Hebron
Which either explains what is meant by the tribes coming to him, (2 Samuel 5:1); namely, coming by their elders as their representatives; or else the meaning is, that the messengers the tribes sent, when they returned and reported the favourable reception they had met with from David; the elders of the several tribes, the princes or principal men met, and came together to David in Hebron:

and King David made a league with them before the Lord ;
the states of the nation; he entered into a covenant with them; he on his part promising to rule them in justice and judgment according to the laws, and they promising to yield a cheerful obedience to him in all things just and lawful: and this was done "before the Lord"; either before the ark of the Lord, as Abarbinel; but that was in Kirjathjearim, from where it was after this brought by David to this city; rather, as Kimchi observes, wherever all Israel, or the greater part of them, were assembled, there the divine Shechinah or Majesty dwelt; so that what was done in a public assembly was reckoned as done before the Lord, and in his presence; or this covenant was made before the Lord, and each party appealed to him as witness of it, so that it was a very solemn one:

and they anointed David king over Israel ;
that is, over all Israel, which was the third time of his being anointed; the first was by Samuel, pointing out the person the Lord chose and appointed king; the second was by the tribe of Judah, when they invested him with the office of a king over them; and now by all the tribes, when he was inaugurated into the whole kingdom of Israel; and not only the elders came at this time, but great numbers of the people from the several tribes, and continued with David some days, eating, drinking and rejoicing, see (1 Chronicles 12:1–40) .

David [was] thirty years old when he began to reign
Over Judah, which was the age of his antitype Christ, when he entered upon his public ministry, (Luke 3:23) ;

[and] he reigned forty years ;
and six months, as appears by (2 Samuel 5:5) ; but the months are not mentioned, only the round number of years given: two reasons the Jews ^F1 give for this; the one, that he fled six months from Absalom; the other is, that he was ill in Hebron so long, and therefore are not reckoned.

FOOTNOTES:

• F1: Hieron. Trad. Heb. in lib. Reg. fol. 77. I.

In Hebron he reigned over Judah seven years and six months ,
&c.] So long the kingdom of Israel continued in the house of Saul after his death; and by this it appears that David was near thirty eight years of age when the elders of Israel came and made him their king:

and in Jerusalem he reigned thirty and three years over all Israel and
Judah ;
which in all made forty years and six months, see (1 Kings 2:11) ; upon his being made king over all the tribes, as soon as he had taken the strong hold of Zion, which he immediately attacked, as follows, he removed the seat of his kingdom from Hebron to Jerusalem.