Therefore I will give thanks unto thee, O Lord, among the heathen, and I will sing praises unto thy name. See Gill on Psalm 18:49.

50 Therefore will I praise Thee, O Jehovah, among the nations,
And sing praise to Thy name.
51 As He who magnifies the salvation of His king,
And showeth grace to His anointed,
To David, and his seed for ever.
The grace which the Lord had shown to David was so great, that the praise thereof could not be restricted to the narrow limits of Israel. With the dominion of David over the nations, there spread also the knowledge, and with this the praise, of the Lord who had given him the victory. Paul was therefore perfectly justified in quoting the verse before us (2-Samuel 22:50) in Romans 16:9, along with Deuteronomy 32:43 and Psalm 117:1, as a proof that the salvation of God was intended for the Gentiles also. The king whose salvation the Lord had magnified, was not David as an individual, but David and his seed for ever-that is to say, the royal family of David which culminated in Christ. David could thus sing praises upon the ground of the promise which he had received (2-Samuel 7:12-16), and which is repeated almost verbatim in the last clause of 2-Samuel 22:51. The Chethib מגדיל is the Hiphil participle מגדּיל, according to Ps. 18:51; and the Keri מגדּול, "tower of the fulness of salvation," is a singular conjecture.