You could estimate the maximum G's your airplane could pull in a loop by doing the following:

1. Find the Clmax of the airfoil your using from a CL vs. alpha graph (this is where the curve peaks)... it should be a number on the order of 1.8-2.2 depending on the airfoil you're using.
2. Estimate the maximum speed of your airplane (this is probably where most of the error in this approximation comes from... picking the speed).
3. Determine the maximum lift force your wing can produce using the following equation:
L=0.5*rho*V^2*S*CLmax
where:
rho is the air density (in slugs/ft^3) (0.0023769 for sea level)
V is your aircraft speed in ft/s
S is your wing area (in ft^2)
CLmax is the maximum CL from the CL vs. alpha graph in step 1.
This should give you the maximum lift force your wing can make in lbs.

4. Take the lift force you calculated in step 3, and divide it by the weight of your model (both in lbs)... The number you get is the theoretical maximum number of Gs your aircraft can pull.

*phew*... I hope I haven't confused you...

Tom