My take on the Pro-Touring vs. G-Machine debate:
Pro-touring is more of a idea of being able to go on long distance runs, through the country side, and so on with all the creature comforts of a modern day supercar, think A/C, leather, multi-media entertainment even. BMW comforts of sorts. The touring is derived from "touring" the roads in comfort while your vehicle is able to handle anything the road throws at you. The "Pro" aspect of the name is derived from the car being able to handle some G-machine duties, a really fun car to do a track day with but it's primary duties are cruising. Pro-touring cars are most likely to be an overdrive or more than 4 manual gears to aid in the selection of the perfect gear. Updated suspension but not necessarily a trick engine combo. It doesn't require pulling at least 1 g but it would require shiny paint job, better than stock handling and bling wheels are a must. Also big breaks are preferred because it fills the space that the large diameter wheels create, it is sometimes done to fit the Pro-touring "look".
G-Machines do require at least being able to pull at least 1 g on a skid pad. It is a no-limits style ride. If it isn't beneficial to the cars performance it is not required. The car will be balanced 50/50 as close as possible. The 17 inch and up wheels are not a must because a 16-17 incher is optimal for cornering. These cars can sometimes have only a drivers seat as they are track only or track/street. It will be a car that is more at home on the track than cruising. The suspensions are usually other than stock unless the vehicle had a really well designed factory suspension that is easily complimented with by high performance components. Exotic rear suspensions usually exist, like triangulated 3 and 4 bars and parralell 3 or 4 bars (not to be confuse with the 4 link wich is more at home in straight line vehicles). Heavily modified engines, big brakes for the performance aspect, and a transmision that gets the job done (they can have anything from 3-speed autos to 6 speed Jericos). They take thier "G" name from the skid-pad and are able to live and thrive in a corner heavy environment. A roll cage and restraining harness is recommend. Good paint is usually seen but isn't always a show quality job. This is open because paint niether helps nor hurts performance. It is not usually a car that you want to go on Power Tour with unless you are a hardcore hot rodder.
These two styles are very similar and often blended together. The result usually falls more on the Pro-touring side though. "Pro-G" if you will.
There has recently been some examples of a style I refer to as Rat-G. This style is a more radical take on G-machine. Anything that is not beneficial to the performance is simply not allowed. These cars will be primered or multi-colored un-finished bodies. They have a drivers seat and sheet metal layden interiors. The owners have the same school of thought as Rat Rod enthusiasts where primer-shiek is cool and sometimes completed in away that looks incomplete.
I currently fall under "non-running Rat-G" but, someday hope for it to be a "Pro-G".
Just my $0.02
