Based on your comments, here is another approach. Define a function
function G = myGfunc(x)
if nargin > 0
This saves the value of G you calculate. Then define
function [F G] = myfunc(x)
G = myGfunc(x);
F = GtoF(G);
After your run, you can enter
and it will simply return the saved value of G.
Caveat: it would be wise to enter
before each run to make sure that G isn't saved from a previous run.