To start with, GameGuru's PBR isn't that great, it's ok, but far from anything other engines can do, it will always look a little dated, to get the best out of them you have to tweak the heck out of the post processing settings, and even then it's not quite right. Max will be much much better, but remember the first alpha in which you can add entities (the other two alpha's are a standalone demo and a basic test of the new engine without the ability to manipulate it) does not have the new engine hooked up, it's basically classic gameguru with a new GUI, and some new features. It will eventually have the "Wicked" game engine which should in theory give us graphics to rival most other engines.
Secondly, take a look at my texture guide for gameguru over on the steam guides, so you can be sure you have the correct naming convention: Modeling For GameGuru: Texture Sets Needed For Entities
. GameGuru uses the metallic/roughness workflow, but with a twist in the naming conventions.
Lastly, i can't remember if i added it to the guide, but the normal map needs an adjustment to get it to work properly in GameGuru. Most PBR normal maps are +Y in the normal directions, however GameGuru needs the green channel to be -Y, that makes a huge difference.
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