What does the Mm and Gm mean?

@NebulaSpaceAgency yes lol

Mega Meters, Giga Meters, probably

@SmurfResearchX ok

yes - i think, in the game
1Mm = 1,000 Km
1Gm = 1,000 Mm or 1,000,000 Km

@DrexxVolv Yes.

@pianoplane They are just giant distances.

@DrexxVolv Okay.

Google:
A megameter (Mm) is a decimal multiple of the base SI unit of length, the meter, which is defined as the length of the path traveled by light in vacuum during a time interval of 1/299,792,458 of a second.
A gigameter is a unit of Length or Distance in the Metric System. The symbol for gigameter is Gm and the International spelling for this unit is gigametre. The base unit for a gigameter is meter and the prefix is giga.