The history of all-terrain vehicles is very long at MB. Their first attempt, in the early 1900 (before WWI) was an extremely innovative all-wheel-drive, all-wheel steered vehicle for military purposes. A dozen of others MB military AWD vehicles followed.
The famous Unimog (UNIversal MOtor Gerät, all-purpose motor vehicle) is not to be forgotten, even if it's not a pure MB product at the beginning.
The idea of the actual G began in the 1960's at MB. The decision to produce it was taken after the order by the Shah of I don't know where, who at the time was an important shareholder of Daimler-Benz, of several thousands of the upcoming vehicle for his army (BTW, when the G entered the market this important order was cancelled by the new leaders of this Arabic country).
They decided to cooperate with Steyr-Daimler-Puch (it seems, because they were impressed by the Pinzgauer, who won at the time a contract over the Unimog for the army of an Europena country).
So they did a Joint-Venture at 50/50.
Steyr only developped the chassis and transfer box. Absolutely everything else came from MB, including the axles for instance.
The G is build in Austria by Steyr-Daimler-Puch, now renamed Steyr. It started its life in 1979, was reworked in 1989/1990. Its name, Geländewagen, means all-terrain vehicle in German.
The G comes with Puch logo and brand on it in Austria and Switzerland (and some East-European country I think), as a MB everywhere else. it has MB engines and gearboxes (bar the Peugeot/panhard P4 with Peugeot engines and gearboxes).
A three-axles G is available for the army.
The contract with Steyr ends in 2010 (31 years birthday of the G), with an option to make it last to 2012. We all hope it will remain a lot longer than that! (the initial contract was due to end in 1989 and resulted in the only big rework of the G).
So the G is a whole, true Mercedes-Benz. Puch only worked on the transfer box and chassis, not even on the axles.