Reinforced Armor Plating is a " promotion" for the Giant Death Robot in Civilization VI: Gathering Storm. Since GDRs cannot gain XP or levels, it is automatically conferred upon every GDR a player owns or builds after researching Smart Materials.
+10 Combat Strength when defending against land and naval units.
Protection and mobility are offsetting factors. You cannot add armor and retain mobility, and the easiest way to improve the mobility of a weapon system is to reduce its armor weight. Even the mighty Gun Deployment Rig must consider these factors. The reinforced armor plating on most GDRs is a composite lightweight armor which adds considerable bulk to the chassis. The gold standard in GDR protection is the "New Mountain Pattern" lamellar system developed in China.