Charles Ellicott Commentary Psalms 138:5

"Yea, they shall sing of the ways of Jehovah; For great is the glory of Jehovah." — Psalms 138:5 (ASV)

In the ways.—Rather, of the ways, as this preposition is frequently used in this manner after verbs of speaking or praising (Psalms 44:8; Psalms 87:3; Psalms 105:2), though there is no parallel instance of such use with this particular verb, sing.

For ways, when used of God’s mighty works in creation, see Job 26:14 and Job 40:19; and concerning His action in history, see Psalms 18:30 and Deuteronomy 32:4. It seems contrary to the parallelism to understand literally that the gentile kings would come to walk in God’s ways—i.e., in righteousness, and so praise Him, as in Micah 4:2. The meaning is that gentile monarchs will be compelled to acknowledge the glory of Jehovah.