Version 1.0.21 can add gloss maps,
but only through the Block Details menu, from what I've seen.
I'm not sure if you've already tried this, but see if you can add the gloss map through the block details in your version too Fallen One (or perhaps update to my version?).
Press F3 to open the Block Details menu, find where it says "Has Gloss Texture", and double click on the "no" in the Value Column, so it changes to yes.
Then extend the "Gloss Texture" part that becomes selectable underneath, so you can see where it says "Source" (for source texture).
In the Value Column it should say None, right click on this and enter the location of your texture file (for example, if the texture is just in the textures folder, just type the name of the texture file in, if it's in a folder in the textures folder, for example, I've put my sky_cr_mino_03.dds texture in a 'dirnae' folder, you'd type in something like dirnae\sky_cr_mino_03.dds)
Hope that works for you too.