aurc89 - find the indices of x1 and x2 within your x vector and then fit the curve against x and y using these indices. For example, if idx1 and idx2 are the indices of x1 and x2 such that x(idx1)==x1 and x(idx2)==x2 then fit the curve between these two points as
fit(x(idx1:idx2),y(idx1:idx2),'gauss2')