A specular map huh? Unfortunately for you, Godot only supports one workflow and that's the metallness workflow. What you would need to do is convert your texture maps accordingly; specifically the albedo and specular maps. According to this webpage you must do the following to convert from specular to metallness.
- Create a new texture. This texture will be your metallic map. Usually it is only either black or white. White where there is metal, and black otherwise. However no one's stopping you from having in-between values for transitions and such.
- After creating that texture, open up your favourite image editor and blend the albedo map and the specular map together using the metallic map as a mask. The result will be your new albedo map.
And that should be it. You now have a modified albedo map with a metallic map replacing your specular map. I'm sure you know where to put them in Godot.