Your question is somewhat similar to the question how to convert mercury-air capillary pressure from MICP to oil-water capillary pressure saturation functions. There are two aspects, first the capillary pressure and second, the saturation. 

For the capillary pressure, you have to multiply the gas-oil capillary pressure by the ratio of interfacial tension * cos(contact angle) for the two systems, 

i.e. Pc(gas-oil) = pc(water-oi) * (IFT*cos(alpha) gas-oil) / (IFT*cos(alpha) water-oil)

You have to be careful, though, with the contact angle, becaue it is usually measured through the more dense phase (which would be ok here because water is more dense than oil, and oil is more dense than gas, so in both cases the wetting phase is the moredense phase), but not everybody sticks to that convention.

For the saturation axis it is more complicated, because the residual saturations might be different. Generally, the saturation axis is the wetting-phase saturation, i.e. If your rock in the oil-water system is water-wet, then water is the wetting phase. However in the gas-oil system, the oil will be the wetting phase because gas is always the non-wetting phase. But you have to make sure that this is really without any water whatsoever, if there is connate water present, then this is a lot more complicated. So you would have to explain more the context of your problem.

Also depending on your situation, the trapping behavior might be different, and hence the residual wetting and non-wetting saturations might be different. One way of looking at it is that as an intermediate step, one can make the pc dimensionless, and normalize the saturation to the mobile saturation.

Pc* = pc / (IFT * cos(alpha))

Sw*= Sw-Swc/(1-Swc-Sor)   

Where Sw is the wetting phase saturation, Swc the irreducible wetting phase saturation (connate water), and Sor the irreducible non-wetting phase saturation (residual oil saturation), for a water-oil system.

So for the conversion from system 1 to system 2, you can say

Pc*1(Sw*1)=Pc*2(Sw*2) 

hope this helps

Dear Dr. Steffen Berg,

It was so helpful. Thank you very much.

Kourosh