How do you fit a gamma distribution?

Expanding on what Wayne wrote, you can supply your fixed-parameter version of the gamma distribution to the mle function. Try this:

x = gamrnd(1.1,100,100,1);
ab = gamfit(x)                                 % fit a and b
b = mle(x,'pdf',@(x,b)gampdf(x,1,b),'start',1) % fix a=1

% compare the empirical distribution and two fits
ecdf(x);
xx = linspace(0,max(x));
line(xx,gamcdf(xx,ab(1),ab(2)),'color','r')
line(xx,gamcdf(xx,1,b),'color','c')