So the very first step in design the OGBM army was to determine if the current rules made it possible to field an army of Ork Gargants and still have a balanced, fun game.
The answer was an unqualified no.
Not, as you might expect, because the Gargants stomped on the opposition but because the test game ended on turn three with a substantial victory for the Space Marine army. Not that the Marines really did that much damage to the Gargants. They managed to break two of the regular Gargants but that was about it. The game was won by possession of objectives and the selfless sacrifice of a formation of Land Speeders that moved into assault range of a Greater Gargant in order to keep it in its own table half for the turn.
Since the Marine player had twice as many activations as the Ork player it was a fairly simple task to take objectives and claim goals. With two of the four Gargants broken t was next to impossible for the Ork player to control any objectives. And with the marine formations running from cover to cover to keep away from the Gargants it was quite difficult to do much damage to the Marines.
The immediate lessons learned from the game were:
Lesson #1: Gargants are big but they still break if you concentrate fire on them. Especially if the Ork player has bad rolls for Power Fields
Lesson #2: Having a lot of firepower doesn’t mean much if you are firing it at small formations
Lesson #3: Having a small number of activations means that you are often outgunned, suffering from crossfir and unable to hold or contest objectives
So the first design goal of the army list was to come up with ways to overcome these issues so that the OGBM army could be playable even if the Ork player chose to take nothing but Gargants.