I am not sure if this is want you are asking, but you can pass a function handle as input. A mini example would be
function y = eval_myfun(h_fun,input)
y = h_fun(input);
end
and then use it as
f =@(x)sin(x);
y = eval_myfun(f,2) % this will evaluate f at x == 2
In your case, you would first generate handle to a purely time-dependent function
h_gauss = @(t)gaussian2(t,tau);
and then pass h_gauss as handle to RMStau.
For RMS however, I don't see why you wouldn't simply do this in two steps.
1. Evaluate function y = gaussian2(t,tau);
2. compute RMS
