What is the gram formula mass of caso4.2h2o?

So your practically trying to finding the molar mass of CaSO₄.2H₂O and that is found by adding all the molar masses of the individual elements:

So Ca = 40 g/mol

S = 32 g/mol

O = 16 / mol

H = 1 g/mol

molar mass of CaSO₄.2H₂O = [ (40) + (32) + (16 * 4) + (2 * 1 * 2) + (2 * 16) ]

= 172 g/mol