The RX-79(G) Gundam Ground Type is a limited-production, ground-based mobile suit developed by the Earth Federation, and were created mostly from leftover parts from Operation V, which spawned the Guntank, Guncannon, and the RX-78 Gundam mobile suits. One of the unique features of the Gundam Ground Type is it's backpack, which has latches that allow it to mount several different types of optional equipment for different purposes. One of these is a large container, used to store several extra weapons for certain situations, such as a 180mm long range anti-MS cannon or a 6-tube missile launcher. (Hence, the "WR", or "Weapon Rack" in the name.) As the GM Ground Type shares about 80% of the same parts with the Gundam Ground Type, it may easily have it's simple backpack swapped out for the Gundam Ground Type's if the mission demands it. This is one of the main mobile suits seen in the OVA Mobile Suit Gundam: the 08th MS Team.
Obviously, the container's original purpose is lost in Gundam Battle Operation, and it only serves as an aesthetic difference to disinguish it from the basic Gundam Ground Type. The Gundam Ground Type (WR) may only use either the 180mm cannon or 6-tube missile launcher as it's main weapons. The Gundam GT WR Type's pre-game lobby icon is a Yellow/Support Gundam head with the letter "GWR" in the bottom left corner.