gamma correction changes the colours - this sort of thing will happen if you enable it for bitmap input/output game engines (as a rule) don't expect to be given gamma corrected textures so if you're making game engine assets you shouldn't have gamma correction enabled.
I know this is an old thread, but it doesn't seem to have an answer for viewing normal maps in the viewport when Gamma is enabled. If gamma is enabled you should Override the gamma setting when you are selecting the file. You also need to plug it into a Normal Bump node.