Change the code like this
g = matlabfunction(realG1)
obj_fun = @(x) g(x(1),x(2)); %<<=== add this line
rng default
gs=GlobalSearch;
opts = optimoptions(@fmincon,'Algorithm','interior-point');
problem = createOptimProblem('fmincon','x0',[0,0],'objective',obj_fun,'lb',[0,0],'ub',[],'options',opts);
[theta,xi,output] = run(gs,problem) % ^ change to obj_fun
fmincon only accepts a function handle with input if your function has two inputs, the use 'x' as a vector with two elements.
