ezplot is for expressions with a symbolic variable. You have a function file with a function in it which returns a value for every y you give it. So what you should do is define some values for y you want to use for the plot and get z from f(y). So:
y=1:100; % 1,2,3... 99,100. or 1:0.01:2 --> 1, 1.01, 1.02 .. 1.99, 2
z=f(y);
plot(y,z)
To use ezplot you can do:
%declare constants (B C D etc)
syms y;
z= (1 + B./y + C./(y.^2) + D./(y.^5) + c4./(Tr.^3.*y.^2).*(Beta + gamma./(y.^2)).*exp( gamma./(y.^2)));
ezplot(z)
