The "Activation Expires" date as reported on the System Activation Summary screen displays invalid after a period of time or a date that is in the past. This date does not affect functionality.
This is a known issue, the CR for this is issue is 47725. This is a cosmetic bug and has no impact to the functionality of the gateway, and therefore has a very low priority.
If you would like to be added to the CR please open a Smart Self-Service Portal and refer to CR 47725 or this document number.