I'm am so new here I hesitate to even chime in, but I have been (and this may not be the right way) adding the mix shader to the output of the principled shader and connecting emission shades to the other mix input with success. I think it shoukd work with a glossy shader as well, but again I am super new here and this is my Virgin answer so I hope it helps.
Not 100% sure what your question is about but I'm assuming you want to make your scene more glossy. (Also, the image is too small. It's very hard to see anything of the node setup)
To add more gloss with the Principled BSDF you would simply reduce the Roughness value. If you have a texture plugged into the Roughness, you can add a Color Ramp in between and make it darker. (I.e. less rough)
Karol, here is a basic node organisation for this kind of texture:
- Use a soft like CrazyBump or other to convert your ground picture to Normal and Specular pictures.
- Load them in Image Texture nodes, choose Non-Color Data.
- Plug the Normal map into the Normal sockets of the Diffuse and Glossy nodes.
- Put a Converter > Math node (Multiply mode) between the Spec and the Mix so that you can control the intensity of the Glossy effect.
same but with the Principled, plug the Spec in the Roughness socket (= Glossy):