DG-68: Can you confirm that the “GATS/M-RETS system registration application and approval letter” is the only supporting document needed for an existing system?

Not necessarily.  The GATS/M-RETS system registration application and approval letter is sufficient to show that the system is existing (although other documentation may be provided to confirm that the system is existing).  However, there may be other documentation required based on the particular characteristics of the system. For instance, a Bidder must provide a Bidder-Owner Agreement for any system (new or existing) for which the Bidder and Owner are not the same individual or entity. More details regarding the documentation requirements can be found in Paragraph IV.3.4 of the RFP Rules.  Furthermore, you can use the Documentation Insert #P1-3 as a guide to the documentation needed for your particular systems.