GIMP is powerful image editor, and it is free. I think only GIMP is the right and free replacement for Photoshop, i am a Linux user, so there is no guarantee i can perfectly run Ps in Linux, even with wine. So i choose GIMP for texturing and other general purposes
Okay, straight to the point, tonight i want to share my own method to generate specular map in GIMP. You can also generate specular map (and other map to, like normal map) by using Insane Bump plugin
First, open your texture or image that you want to generate specular map from.
Second, desaturate the image by using Color > Desaturate , and select average button. But you also can try other type like lightness or luminosity
Third, on the right side of your GIMP, click on “Duplicate Layer” icon(1), and with duplicated layer still selected, click mode menu(2) and select Dodge mode
Fourth, with still same layer selected, duplicate layer as many you want and find optimum duplication, so the specular map looks good ( in this screenshot, i duplicated the layer 13 times)
The last, flatten the image by using Image > Flatten Image menu, so you will only have one layer now. The specular map now is ready to save or export
Now, your specular map is ready to use.
Thanks for reading my simple tutorial, i hope this useful