The original design intent was to keep GIM-specific data separate from XMC data. This is why both users and devices will be tagged with a "Managed by GIM" description (if it's not in the release you're running currently, it will be in an upcoming release). If a device or user is "Managed by Gim," the understanding - and deployment scenario - is that it will mainly be managed by GIM (either an admin or provisioner), and not by XMC.
That said, for devices, specifically, there is a difference between a DEVICE and an END SYSTEM. When an end system authenticates to the network, a number of things happen that automatically modify the states and information about that end system. The GIM data for any specifically configured device is checked upon device authentication (i.e., do we see a device manually added by GIM and, if so, do we need to enact any specific actions on it (i.e, change it's OS based on precedence (GIM is considered "manual set"), reject it based on expiration, as examples).
The custom information that you see in the end system table is - at least right now - not shared between GIM and XMC. The "Custom" fields you see for the end systems can be either imported or manually modified by an XMC admin but, again, end system data is separated for the most part from GIM devices, so those fields are not shared.
The functions and interactions between GIM and XMC may (and most likely will) evolve as use-cases come in, and your observation and query has been sent to product management. If I can get you a better answer, I will, but right now what you're looking for is not currently possible.
