As mentioned above it is due to a policy that still references the old gateway. When I ran into this issue, removing a IaaS Gateway, I eventually found a rule that still was being installed on the old Gateway within Mobile Access. The Mobile Access blade threw an error while I was trying to remove all blades when I first received the (gatewayStaticprofilesConfiguration ->Assignment Profiles) error.
This occurs because under Threat Prevention there is a default Global Exception that ties these shared policies to all rules.
Global Exceptions