  We present a numerically stable Quantum Monte Carlo algorithm to calculate
zero-temperature imaginary-time Green functions $ G(\vec{r}, \tau) $ for
Hubbard type models. We illustrate the efficiency of the algorithm by
calculating the on-site Green function $ G(\vec{r}=0, \tau) $ on $4 \times 4$
to $12 \times 12$ lattices for the two-dimensional half-filled repulsive
Hubbard model at $U/t = 4$. By fitting the tail of $ G(\vec{r}=0, \tau) $ at
long imaginary time to the form $e^{-\tau \Delta_c}$, we obtain a precise
estimate of the charge gap: $\Delta_c = 0.67 \pm 0.02$ in units of the hopping
matrix element. We argue that the algorithm provides a powerful tool to study
the metal-insulator transition from the insulator side.
