This mod is largely based off of Monstro Mode, which is based vaguely on True Greed Mode. However, this replaces the enemies with either Gurdy Jr.'s or Gurdy's.
This mod can be easy or hard. Usually no matter what, it is lengthy. So be prepared for a lot of waiting at times!
Chaos is also added to the pools which can make the devil/angel deals pointless but it does make the treasure rooms much more varied and possibly overpowered.
Good luck!