I have found a solution. Upon reading the MSDS of both products i have found this...
Gypsum has the composition of:
90-100% CaSO4 (why its not 99-100% is do not know)
The Plaster of Paris has the composition of:
25-85% CaSO4 and
The conclusion to my findings is if you are looking to increase the kH and Ca content then use the Plaster of Paris. However if your looking to increase just your Ca then use Gypsum.
Hope this helps...