The percentage of freeloaders is something like 33% according to BearShare's statistics, so this is not a serious problem at the moment. You might want to try reducing that figure by adding some message to the GUJI telling users to share files but credit systems are not the way to go.

Not only are there more immediate solutions to increase the performance of Gnutella, - credit systems as a whole are not very effective in an open network. I don't want to go deeper into this topic because it is fairly late and I'm watching a movie, but I will say this much: The probability of meeting the same client twice within a short amount of time is very low in a large network. You would have to handle a huge database of information on who downloaded / uploaded how much. The number clients that would effectively ever be able to benefit from those credit systems would be very small.

The credit system would just produce an illusion of fairness, maybe have some users share more files because they think it allows faster downloads but you wouldn't even be able to measure the effects reliably.