Allow me to break down the difference between changing your gamma and regedit values.
Gamma is within Minecraft. It's a part of the game.
Regedit is within your computer. It's something that you can modify however you want, in whatever ways you want.
Changing your regedit values is like having a tech guy come over to your house and make your computer more efficient.
Changing your gamma is, in essence, like learning how to do very basic coding and making another version of FullBright Mod. (And that is the same as simply downloading and installing FullBright mod itself)
MCGamer does not allow modded clients (aside from the listed exceptions in the rules).
However, MCGamer cannot at all prevent their players from tinkering with their own computer. It's their personal property, and changing the regedit values is like buying a new computer with better specs in order to get higher FPS.
There's a difference, and that difference means a lot when it comes to whether something is or is not allowed.