Form my calculations you pond is no more the 0.05 acres now using that as our size and 6ft as an average depth we would come up with 0.28 acre feet. which if we use Agents 450lbs per acre foot then you would need 128lbs of gypsum but since your pond is smaller then 0.05 acres being that it is not completly rectangular you could exspect to use around 100lbs of gypsum with Agents numbers. im not an gypsum exspert iv never actually used it so idk what good numbers are but im sure someone could give you good numbers for different situations.

0.22 acre dam pond LMB, BG, and CC