John Gill Commentary Psalms 136:26

"Oh give thanks unto the God of heaven; For his lovingkindness [endureth] for ever." — Psalms 136:26 (ASV)

O give thanks unto the God of heaven
the Maker of it, in which the glory of his wisdom and power is displayed; the possessor of it, where he dwells, has his throne, and keeps his court; from whence all blessings, temporal and spiritual, come and where he has prepared glory and happiness for his people hereafter, a house eternal in the heavens, an inheritance reserved there, a better and a more enduring substance;

for his mercy [endureth] for ever ;
for though the above character is expressive of his sovereignty and dominion, yet he exercises it in a way of grace and mercy to the sons of men; and therefore they have reason to give thanks unto him and praise him for his kindness and favours shown to them on earth.