I read that the G's magnesium arm can work with cartridges up to 19.5 grams when using their "additional" counter balance weight.
Double check the above, as I cannot find the source.
I'd weigh the stock headshell (guessing it's 7-8 grams) and then go from there with the weight of whatever cartridge you are using.
Also, was it supplied with the additional weight?
DeKay