Just figured that the gate you mention is probably the one that requires you to solve a puzzle to open. I didn't understand that puzzle at all. There is secret lever (need character with decent points in Detection) above the stone bench just to the right of that gate, if you want to skip it. To solve the puzzle (which gives Sage background Tav or Gale an inspiration point) you should do a google search with this seach term: Baldur's Gate 3, Defiled Temple moon puzzle. Should be a link to a Polygon article. That's the solution to the puzzle. Either option opens the gate.
