Albert Barnes Commentary Psalms 138:1

"I will give thee thanks with my whole heart: Before the gods will I sing praises unto thee." — Psalms 138:1 (ASV)

I will praise you with my whole heart - Reserving nothing in my heart to give to idols or to other gods. All that constitutes praise to God as God, he would address to him alone. He would use no language, and cherish no feeling, which implied a belief that there was any other God; he would indulge in no attachment which would be inconsistent with supreme attachment to God, or which would tend to draw away his affections from him. See the notes at Psalms 9:1.

Before the gods will I sing praise to you - The idols; all idols; in preference to them all. This does not mean that he would do this in the presence of other gods; but that Yahweh should be acknowledged to be God in preference to any or all of them.