Hi Daniel,
Good to know. It's a pleasure to help out.
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(Please correct me if I assume wrong approach) In case VS1 goes down on a LTM cluster (LTM and GTM are on separate boxes right?), GSLB (both GTM's) does mark VS1 as down based on iQuery communication (wihin the monitor "bigip"). Therefore GSLB does not respond the WIDE-IP pool members IP address for VS1 (VS1 as WIDE-IP pool member) for the corresponding request and it will respond an IP from one of the remaining (online) WIDE-IP pool members which should represent the same application logically.
Background info: Basically, a WIDE-IP has as a pool assigned. This pool contains LTM Virtual Servers (you configured manually) from different sites to control traffic within same application. Configuring GSLB methods is per WIDE-IP. So, you decide on a per WIDE-IP basis how traffic should be handled (which is very flexible btw.) across your sites (data centers).
- I would recommend using DNS-Express for this use-case basically. As you can see from the following link https://support.f5.com/kb/en-us/solutions/public/14000/500/sol14510.html how the BIG-IP DNS module traffic is flowing through the box. Use our onbox "bind" which is called "Zonerunner" to configure your zone but expose all the configured zones via "DNS-Express" only. Therefore you need to make SURE you configure whin the DNS profile GSLB, DNS-Express only and make SURE to disable "Use BIND on BIG-IP".
Background info: Basically, you offload all your zones from Zonerunner and AXFR all these into our physical memory (via DNS-Express) on the box itself (which is very fast).
This is just the way you could go for in case you completely want to get rid off your actual DNS. Otherwise I would recommend you to use at least one master DNS and offload the zone to this one. DNS-Express aka DNS-X is used to hide your DNS infrastructure and therefore you don't have to take care of related vulnerabilities for this offloaded systems.
Does all this make sense to answer your questions?
Cheerio,
Andrea