A.T. Robertson Commentary Romans 15:9

"and that the Gentiles might glorify God for his mercy; as it is written, Therefore will I give praise unto thee among the Gentiles, And sing unto thy name." — Romans 15:9 (ASV)

And that the Gentiles might praise (τα δε εθνη δοξασα). Coordinate with βεβαιωσα and εις το, to be repeated with τα εθνη, the accusative of general reference and τον θεον the object of δοξασα. Thus the Gentiles were called through the promise to the Jews in the covenant with Abraham (4:11f.,16f.). Salvation is of the Jews. Paul proves his position by a chain of quotations from the O.T., the one in verse 9 from Ps 18:50. For εξομολογεω, see 14:10.

I will sing (ψαλω). Future active of ψαλλω, for which verb see on 1 Corinthians 14:15.