From Rare Gaming Dump

The GameID on Nintendo platforms is a sequence of 4 (6 with company code) characters derived from the end of a title's Title ID, used to represent that title in an abbreviated form.

An example of a Wii disc title GameID would be "RMGE01" for Super Mario Galaxy, with "R" representing an early-run Wii disc title, "MG" being the unique game code, "E" representing the Americas region, and "01" being the company code.

On the GameCube and Wii, its structure is as follows:

First character Code representing title type (e.g. G for GameCube titles, R (Revolution) for early Wii disc titles, W for WiiWare titles)
Middle two characters Unique title/game ID (e.g. "MG" for Super Mario Galaxy)
Last character Region encoding (e.g. E for Americas, J for Japan, P for Europe, K for Korea, A for none, some titles like the Homebrew Channel don't use this for regions)
Subsequent two characters Company code (e.g. 01 for Nintendo). This is not present on the DS family of systems.