Use Blender Cuda Build to use GPU in Simulations: Ok, my current situation is this: 
I have a very slow CPU (AMD Athlon 750X (2 Cores 4 Threads))
But I have a pretty fast GPU (NVIDIA GeForce 1080)
The GPU actually supports CUDA 6.1

I need to bake some physics simulations, and I tried with the default settings.
It is very very slow Baking, (like 4 hours for a 92 divisions 1 Smoke High Res for 600 frames). I tried looking for a way to use GPU in baking online but I have found nothing except for this:

http://graphicall.org/444

This is the Modified build of Blender 2.78.5 that uses CUDA for most of the things. It says that it sopports both Smoke and Fluid Simulations. (It says Elbeem Modifier (Fluid Simulation), Smoke Modifier (Smoke Simulation) and Tile based nodal compositor in the section Features enabled in compilation). And the file is also called 12348_blender-2.78---6e90294---cuda-and-osl---fastest.7z so I assume it make osl work with GPU rendering. I tried installing it and nothing. I could not get any performance boost in any feature. I also could not enable OSL in GPU COmpute options in Cycle (The Open Shading Language checkbox is greyed out).

I tried searching online for a fix but I could not find anything. I could not even contact the author cause there is no way I could find to contact him.

Is there any fix?

Am I doing something wrong?

Should I uninstall previous Blender Version?
I think you've misunderstood the description for that build. It is merely stating what features were enabled at compile time. It's possible to build Blender without some things, such as the smoke simulator, or without CUDA. The description merely says that CUDA Cycles and the smoke simulator are both enabled, and that the build is speed optimized. It does not mean that the smoke simulator uses CUDA. Or anything else that doesn't normally support CUDA. 

Compiler settings cannot add GPU compute support to existing code. If someone says they "compiled with GPU support enabled" that means the code already supported it, they just compiled without disabling it.