ANSWER: Depending how long your bike is either one of our VH-55 RO or the VH-SPORT RO carrier will work great with your vehicle. The difference between the 2 carriers is the lengths and widths of the rails and ramps.
The VH-SPORT has a longer and wider rail and ramp and has a heavier carrying capacity as well.
A VH-55 RO or VH-SPORT RO are our best carriers.
Please see www.versahaul.com for other vehicle and hitch limitations that may apply.
http://www.versahaul.com/vh55ro.php
Based on the information given I recommend the G500’s carrying capacity has been estimated at 310lbs. Your vehicle without can have 310 lbs motorcycle on top of the carrier and your vehicle will sit level. Additional, weight will gradually compress you suspension. If the truck has air bags or air ride then your suspension rating would be much higher than estimated. Otherwise you might see the rear suspension of your vehicle compress.
The a motorcycle that weighs 320 lbs will exceed the estimated carrying capacity for your suspension for a level vehicle by 10 lbs. Likely there will not be a noticeable effect, but with every lb that you exceed the weight limit the compression will compress.
Please be mindful that most class III hitch are rated for 500 lbs.