In general, assume everything is sRGB 2.2 unless stated otherwise. Your existing color textures are most probably gamma corrected to sRGB because a) that's what the file format standards are and b) the content creator used a monitor with a roughly 2.2 gamma curve.

With post-render gamma correction, I'd also use 2.2. That Mac vs Windows gamma is just a very vague rule of thumb; barely anyone's using a calibrated monitor anyway, so the actual gamma of the screen that it will eventually be looked at varies.