There are 3 flavors of G automatics 720.1 (1981-1990), 722.3 (1989-1997), 722.6 (1995-present).
Here is the dirt on these 3:
1) In 1971 mercedes began production of a 4 speed automatic with torque converter, they also made a 3 speed version for a couple of years which was exclusive to powerfull large displacement V8 cars.
In 1980 MBZ extensively modified this automatic transmission to suit the G and van platforms by integrating an electricaly activated solenoid which keeps 1st gear engaged, no passenger car automatic can hold 1st gear. Transmission case and bell housings have breather 'snorkels' for water fording, deeper oil pan ensures constant oiling at extreme angles and internal components are more robust with extra friction discs, special valve body etc. due to the G's weight. They called this the 720.1.
The torque converter has a torque multiplication factor of 1.8 or 2, so first gear ratio (normally 4:1) is nearly 8:1 under 1800 RPM.
2) 722.3 may be the best non electronic automatic Mercedes has made. Smoother shifting, slightly more 'luxurious' in feel than the previous unit. G-specific versions have the same special features listed above, like the 720.1 it is a vacuum controlled 4 speed torque converter transmission but optimized to compliment the more efficient "modern" engines emerging out of MBZ in the early/mid 80's.
Mercedes used this auto behind its V12 luxo-barges with great success, every 722.3 is easily modified with V12 bits (friction discs/bands etc.) to survive well over 700 horsepower.
*The operators manual for both G-wagen and passenger cars say you can pull start both of these 4 speed autos (720.1, 722.3), and I have tried this on a couple different vehicles, but in my experience this is not practical. MBZ says to get the vehicle up to constant 40kph on flat ground in neutral (builds up pressure in ATF because rear pump always spins) then drop into gear and hope the TC locks up and rotates the motor. There is a risk of over-pressurizing the ATF and popping seals, keep it under 40kph!!!
3) 722.6 or NAG1 is an electronically controlled (CAN-BUS) 5 speed torque converter transmission with overdrive (0.83:1). It is THE best automatic MBZ has made, and it may be the best in the world. Super reliable, seamless gear changes, nearly indestructible, and it can be modified (with AMG/V12 bits) to handle serious HP.
From what I have seen the 722.6 found in all G-wagens is physically and functionally identical to any/all passenger car NAG1/722.6, the only notable differences I know of lie in the number of friction discs, small displacement vehicles have fewer discs then large displacement vehicles.
Holding first gear is now the job of the TCM (transmission control module). There are 2 aftermarket TCMs available that I know of which would allow swapping this gearbox into anything, however these TCMs are not yet plug and play, the end user may need to fine tune shift points/pressures/speeds.
There are other automatic transmissions (722.4, 722.5, NAG2) that I have not mentioned above. These are smaller/weaker and intended to be used in light passenger cars, definitely not suitable for a G.
The 7G offered in modern luxo-Gs was known to be problematic but seems to be getting more reliable as production goes on.