Editing something in your option files is technically a modified client? Then Changing my render distance or FOV in my options file would be modifying my client as well.
The ability to change your gamma is built into vanilla Minecraft.
Considering the Admins have deemed it acceptable to have bright textures its apparent that having increased gamma should also be allowed considering they do the
exact same thing.
Default MC on Moonbase9:
Now look at the following two images. One of which has increased gamma (editing through the options text file) And the other being a custom Redcraft texture pack.
It's impossible to ban someone for increasing their gamma now unless you see them in their video settings and see 1000% brightness.