Specular texture

[kulharin]

If I only have a diffuse and a normal map, is there a filter or tool in GIMP that would help me create an appropriate specular map?

[kulharin]

For grass, plants and vegetations specifically

[denzjos]

Use brushes, you can download some here : https://www.brusheezy.com/free/grass
How to install : https://www.youtube.com/watch?v=JC_tWVHMzVA

[rich2005]

I am no expert but you might get something by desaturating the diffuse map. A big perhaps, all depending on image, and I think not possible.

   

Original from Gimp using mypaint brush, Maps made using the ancient insanebump plugin.