I need to measure 1-5G Ohm resistors. Is Wheaston bridge suitable for this?
No, according to my opinion it doesn't suitable in this case. Simply use a current measure circuit instead, using a relative high measuring voltage, for example 100 V if possible, so you can measure 100 V/5 GOhm = 10**2/5*10**9 = 2*10**-8 = 20 nA for 5G (and 100 nA = 0.1 uA for 1G) !!! I think this is possible.