I should pass this model on Unity. I think the only way is to bake the material. I can not proceed because of the “metallic”. The glossy bake is always white. Why? Where am I wrong?
Thanks in advance
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Baking in Blender means baking the lighting information to texture maps, not the properties of the Principled shaders like roughness. You can bake AO and normals, but as far as I know, currently Blender Cycles only bakes normals from geometry and combining bump maps into a normal map or a few normal maps into one will not work correctly.
Buzzwords like 'PBR texture' or 'PBR baking' do not change the fact that this is nothing more than a process of making texture maps for a material. 'PBR' may only indicate that the values in the texture maps are scene referred(realistic reflectance values), that's all.
So I believe your goal should simply be to make textures for your material in Unity. You can experiment and see preview of the material in Blender using Principled BSDF. It will be easier if you work with only one Principled BSDF node and combine/edit the needed textures for it's properties. Whatever textures you combine or edit in the node tree of the material can be connected to an emission shader and baked out as emission:
You would just take the output of the last node just before it goes into some property of Principled BSDF shader(like for example Roughness) and connect it to an Emission shader and bake it. This is how you would make a single texture out of everything that you combine together in the node tree for that property.
You probably do not want the values to go through any color management transforms, so you can turn color management off during this process:
Baking ambient occlusion might also be useful, but it depends on how you want to set up the shaders in Unity. It can be made into a separate map, or mixed into other maps depending on the situation.
Bump maps can simply be mixed together(added, multiplied, overlayed or mixed any other way that produces a desired height map).
If you needed to combine a few normal maps on top of each other, that is a lot more complicated than it may seem - see this answer for more info.