There is a relevant thread here:
Why did Hitler never use biological or chemical warfare? - World War II Forums
That Hitler did not use Tabun and Sarin against his non-Geneva Convention opponents, the Russians at the end was, IMHO, due to the lack of means of delivery and a possible lack of access as Germany was squeezed on two fronts.
Roosevelt would seem to have been the controlling factor on the allied side holding the chemical weapons as a retailiatory weapon. Of course his decision of first use of atomic weapons is counter to that. Maybe he had experience of chemical weapons but only theoretical knowledge of atomic ones and no concept to the actual effects.