Yes, the chemicals used in the tanning process can damage any metal coming into contact with it. The best storage is an acid free shoe box with acid free tissue between the pistol and the holster.
However, like others here, I have used RIG for 35+ years and have never had an issue with any firearm slathered in RIG. This is especially true for Randall tool-steel knives which will stain with time in the sheath without RIG.
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