From Dragon Ball Encyclopedia, the ''Dragon Ball'' wiki
Planet M2 (エム二 惑星, Emuni Wakusei; FUNimation "Planet M-2") is a planet in the Northern Galaxy.
Dragon Ball GT
Son Goku fought against Rild, the Sigma Force, and many other Machine Mutants on Planet M2. Many areas of the planet were wasteland, but there was one huge city. The residents were all robots or Machine Mutants. Gill guided the group around this planet as the plot that revolved around his loyalty unfolded. This was also where Baby was first confronted.
It is also a battle stage in Dragon Ball Heroes.