Use sRGB Color Space
This options enables sRGB gamma. sRGB gamma cannot be expressed as a single value. Due to that, you can’t control its value in gimp-dds plugin. I’m not going to delve into the details of sRGB color space. For more information about this topic, please refer to Wikipedia.
Lets you specify the gamma value. Default value is 2.2.
Preserve Alpha Test Coverage
Standard mipmap algorithms produce very bad results on alpha tested textures. This options solves this issue by choosing a suitable scale factor that preserves the original alpha test coverage as best as possible. For more detailed explanation about this problem and possible solutions, please refer to Computing Alpha Mipmaps tutorial.
Alpha Test Threshold
You should use trial and error to find the best alpha test threshold to fix the aforementioned problem.