G is growing in the loop, so Pin(z)*G is a vector (for m >= 2). You can't assign a vector to the single value Pout(m), hence the error.
Perhaps you mean Pout(m) = Pin(z)*G(m)?
Also, you should preallocate these arrays, so they don't grow inside the loop:
tlen = length(t);
G = zeros(1,tlen);
Pout = zeros(1,tlen);
etc
EDIT TO ADD: Actually, assuming you are just trying to do element-by-element calculations here, you don't need to do all these in the for loop. The quadrature is the only thing that needs a loop, so try this:
t=-3:.1:3;
Go=1000;
Bc=5;
Pin=@(t)3*exp(-t.^2);
Ein = quadgk(Pin,-inf,inf);
for m=1:length(t);
z=t(m);
E1=quadgk (Pin,-inf,z)/Ein;
end
G=Go./(Go-(Go-1)*exp(-E1*0.1));
Pout=Pin(t).*G.*1;
Phi=-0.5*Bc*log(G.*1);
Aout=sqrt(Pout)*exp(1i*Phi);
Aoutf=fftshift(fft(Aout,100000));
f = -(-100000/2:(100000/2-1)).*1/(0.01*100000);
Poutf = abs (Aoutf).^2;
plot(f,Poutf,'--')
xlim([-15 5])
