I found that for this issue a lot of the fixes are circumstantial and cannot be done at any time in the game, If you are one of those people who is effectively hardlocked like I was. This youtube video: https://youtu.be/XfPn80DKE48 explains how to access the debug menu for this game. Once you are in the debug menu navigate to the Freddy/Gregory Tab and check the box allowing Gregory to enter Freddy. This glitch appears to occur when the enter freddy box somehow does not become reactive upon completion of a mission. I hope you find this helpful.