Sync Only config for HA and standalone BIG IP devices

A Sync Only group will synchronize your APM policies as well as any ASM policies, but will not synchronize any of your traffic objects (Virtual Servers, pools, etc). If you setup a Sync Failover group, it will fail over to the standby just as it would if the device was local. This is likely not what you want.

You could configure the GTM to only use the secondary data center if the primary is offline, which is likely what you want to do. The caveat there is that you will need to have the GTM configured in the secondary data center to pass results to the world when the primary data center goes offline.