Not sure I fully understand the request - you would like one gold object to show no reflections of the surrounding 3D scene, but only show an HDRI in its reflection? If so, the way would be to do two renders, one of the scene without the gold object, one with just the gold object with the rest of the scene hidden or deleted, and then composite in post. While you can easily override the HDRI seen in the reflection on the gold object, there's no way to prevent that object reflecting the rest of the modeled scene around it (just like in the real world), so two-renders-composite-in-post would be the way to go.
(Well, you could apply a RaySwitch material to EVERY other object in the scene, where the Reflection slot has a material with 0% opacity which would make those objects invisible in reflections - this might be possible if the rest of the scene is being painted with one material, which may be the case here as you mention something about the rest of the scene being gray, but would be super hard to set up with hundreds of materials otherwise... plus those objects then wouldn't reflect in each other either, rather than just not reflecting in the gold object)
