This happens on:
Minecraft 1.15.2
LWJGL 3.2.2
Fabric 0.8.2+build.194
tweakeroo-fabric-1.15.2-0.10.0-dev.20200320.230739.jar
malilib-fabric-1.15.2-0.10.0-dev.20+arne.6.jar

Description:
tweakGammaOverride is ignored when I change the Minecraft brightness settings in options, which means the Gamma turns back to normal. The current workaround for this is to turn tweakGammaOverride off then turn it back on every time the brightness is changed. This applies to both with and without Optifine.

This is because currently all the gamma override tweak does, is set the vanilla value when you turn the tweak on (and also when you start the game with it on). This was done to avoid doing more or less unnecessary bytecode editing for this feature. But I can check how many places the value is used in and whether or not I could easily switch into using Mixins to actually override the value instead of just setting the vanilla value.