John Gill Commentary Psalms 118:29

"Oh give thanks unto Jehovah; for he is good; For his lovingkindness [endureth] for ever." — Psalms 118:29 (ASV)

O give thanks to the Lord, for [he is] good
And thus the psalm ends as it began; there having been given many instances of the divine goodness, in hearing and delivering the psalmist when in distress; saving him from his enemies, when surrounded by them; sparing his life, when in great danger; and especially in making the stone rejected by the builders the head of the corner;

for his mercy [endures] for ever ;
the above instances are proofs of it; and still it continues, and will for evermore. Here ends the great "Hallel", or hymn, sung at the passover and other festivals.