Gallo is not a PG. He can handle a little bit, but he looks bad trying to orchestrate. He is better as a 2-guard/combo guard who catches and shoots off of ball movement, or finishes at the rack when the opportunity presents itself. Yes his overall man D is superior, but the offense would stagnate even more if he were the primary initiator. Calderón makes better decisions with the ball and gets other guys quality looks much more proficiently than Gallo has ever shown he can.
We are thin at PG no doubt. Grant is not ready and Calderón's D can be lacking. However, over the last several games Calderón's defensive effort has been decent and his overall D has not been a main reason for the Knick losses. In fact he has been pesky, gotten a few steals and generally been fine. He is clever and protects the ball well and moves it to the right spots, makes few mistakes when running the offense. Not flashy, but effective in a quiet way.
That said, an upgrade at PG should be at the top of PJax's list.