John Gill Commentary Psalms 106:21

"They forgat God their Saviour, Who had done great things in Egypt," — Psalms 106:21 (ASV)

They forgot God their Saviour
Not only forgot the works of God, (Psalms 106:13) , but forgot God that did those mighty works; forgot that there was a God; or however forgot him that is the only true and living God, when they said of the molten image, "these are your gods, O Israel"; and what aggravated their crime was, they forgot that God that had saved them in so wonderful a manner out of the hand of their enemies, for his own name's sake; and had led them safely through the Red sea as on dry land, and destroyed those that hated them.

Which had done great things in Egypt :
meaning the ten plagues on the Egyptians, and the deliverance of the Israelites from them.