Continuing to assume for the sake of example that G = 5G at Jupiter (or G=10G...whatever you want), and assuming a radius of approximately 68,000km for Jupiter, can you show how you would calculate the acceleration due to gravity at 68,000km from the center of Jupiter (ie: surface gravity)? I'm interested in the steps you would take to measure the mass of Jupiter (and what value you think that should be), and then the steps you would take to use that mass to determine the acceleration due to gravity.

Perhaps working through an example would help us understand what you are trying to say.