To get the 75th percentile value, you need to use gaminv() rather than gamcdf. Also, the shape parameter comes first and scale second. So, you want
gaminv(0.75,1,2)
% ans = 2.7726
With a shape parameter of 1, this is the same as the exponential distribution:
expinv(0.75,2)
% ans = 2.7726