How do you apply a gloss map? I've searched all over the internet for this and nothing has come up. Everything is about spec maps which I don't want.

I would like to know how to apply a gloss map to a material in Blender Cycles.

I have a marble texture with an image texture input to a diffuse texture, and a glossy shader with a white colour. Both are being put into a mix shader.

I have downloaded a gloss map but I don't know how to apply it.

Any help would be greatly appreciated.

Thanks in advance.
A gloss map is a map of specular power, the inverse of a roughness map.  Read it as non-color, then subtract it from 1 (but see below) via a math node and plug it into the roughness value of any/all shaders.

There is no single, standardized format for gloss/roughness/specular power-- there are instead a lot of different ways to map from color to gloss, depending on engine.  So in order to use a gloss map correctly, you have to know the engine for which it was made, and then you have to know how that engine interprets gloss.

Or you could just wing it.  Start with the simple inversion I mentioned above.