Steps to reproduce:

1. Create a new unity project
2. Add a plane to the scene (GameObject -> 3D Object -> Plane)
3. Create new material (Assets -> Create -> Material)
4. Set the material color to simple gray (R 128, G 128, B 128)
5. Attach the material to the plane
6. Make sure 'Color Space' property is set to 'Gamma'
7. Change the 'Color Space' to 'Linear'
8. Notice how material color is set to brighter gray (R 188, G 188, B 188)
9. Set the 'Color Space' to 'Gamma' again
10. Notice the color goes back to 128, 128, 128
11. Set the 'Color Space' to 'Linear' again
12. Notice how this time material color stays the same (128, 128, 128)

Note: If you change the color of the material after the first switch to linear and then go back to gamma space, it will stay at the value that was set manually.