Specular map not working no matter what I do...

I've watched so many tutorials specifically on creating specular maps for The Sims 4 and where they go in The Sims 4 Studio and what all the different colors mean, but no matter what I do my specular map doesn't affect my item in game. I must be messing something up because I'm using Gimp and the tutorials I've seen are in photoshop. I'm exporting the file as a .dds, I have an alpha channel, and no matter what I do to the specular map the items don't change appearance from one map to the other...

I'm making an end table with a marble top and a metal stand. I'd like the metal stand to be really shiny and reflective, and the marble top to be a little glossy. The marble top currently looks very shiny in game and the metal stand is shiny but not reflective...I'm so lost :(