To be honest I can't remember if I have posted this before, but people have been asking for it.
I have included my "Apply Image" script that makes a 1:1 highpass of your texture. This can be used on any texture, I mostly use it on diffuse and glossiness to make real time adjustments in Corona.
All the blending modes are in sRGB! (Something that is Corona exclusive for now!)
Open your texture map in Photoshop and run my attached script.
You will get this result.
Hide the bottom layer "albedo color" and save the "apply image high pass" map as a png or tiff.

Create a setup like this, where you sample the bottom layer as "Albedo" color.

You can make a glossiness variant like this
But I usually normalize my glossiness map in Photoshop.

Just remember that normal materials does not go bellow 0.4 glossiness. It is great that we have the opportunity to go down to 0. But just keep in mind that everything bellow 0.4 is for special occasions. If you have to fake depth, use an IOR map (Reflection Level clamp your IOR).
If you don't understand how how reflection level work, think of it like this. You have a black background layer in Photoshop, you have your IOR as a white layer above. If you reduce the "reflection level", its like you are reducing the opacity of the white layer in Photoshop. 89 degree will never be white!