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Hello audimar !
GG has a PBR rendering pipeline and reacts accordingly (ish).
What you need to do is this: Close GG, open the .fpe files and look for materialindex.
Here you can assign a material.
"0 = GenericSoft
1 = Stone
2 = Metal
3 = Wood
4 = Glass
5 = Liquid Splashy Wet
6 = Cloth / Flesh / Soft Absorbant (blood effect)
7 = Hollow Drum Metal
8 = Small High Pitch Tin
9 = Small Low Pitch Tin
10 = Silly Material
11 = Marble
12 = Cobble
13 = Gravel
14 = Soft Metal
15 = Old Stone
16 = Old Wood
17 = Shallow Water
18 = Underwater
"
2 = metal is shiny for example and so is 4 = glass.
Also make sure that every entity has a shader assigned to it. Add this line to "effect" -> "effectbank\reloaded\entity_basic.fx"
You also want to make sure that all your diffuse textures end in _D.dds
Hope this solves your issue!
Cheers
-Wolf