GTM Device groups

The HA configuration "local" has no relation with GTM Sync Group. You can have an HA configuration,... active/standby, active/active.

GTM is always standalone (and implicitly active). If you have more than one GTM, they will work together. For example, splitting work (like monitor), and sharing data (DNS zone files, WIPs, DNS pools).

When you have LTM and GTM together, LTM will still work independently from the GTM, but you can configure the GTM to take advantage of "local" HA clustering.

The GTM listener is a LTM virtual server for all intents and purposes. If you use the floating IP, only the active unit will respond to queries. If you configure the GTM devices with Listeners using the non-floating self IP, both can process DNS requests. Keep in mind the last scenario means that the DNS clients will have both IPs configured for DNS Servers.

More simply put, the HA "local" clustering, and the GTM Sync group clustering are completely separate.

Based on your overview, I would think you would want two HA clusters with one listener each. And all 4 GTM devices, in a sync group, sharing all DNS information (Bind, WIPs, GTM Pools,,.. and so on.). Also, depending on how you monitor the resources, use prober pools to keep the heavy lifting localized.

Hope it helps!

Good luck!