@Amr_Ali Do you mean having an HA device much like you have HA LTMs? If that's a yes you really don't have to do that because GTMs are inherently redundant for DNS because of how the sync works between them for DNS configuration. You might need them to be HA because they run multiple functions such as GTM, LTM, AFM, and so on but you should try and keep GTM off of the same box as those other services if possible.
I agree with @Paulius 100%. If the F5 BIG-IPs only running GTM as dedicated module, no need for HA! Use GTM sync group and publish both listener addresses for DNS... or setup DNS anycast. If in HA, you don't have BOTH units responding to DNS (normal setup), but if both are standalone and part of the same GTM sync group, then you still have resiliency and depend on iQuery between both GTM instances and DNS for failover. If running more modules... like LTM and/or ASM the conversation changes. Most clients with run dedicated smaller devices for GTM/DNS only, and a bigger device for all other ADC functions.
the sync group is just to make sure all configs are synced when changes are made in the sync group. The individual GTMs will get requests based on the NS records established on zones. If you really want only one to respond, then HA is the way to go, but I'm curious to understand the use case of sidelining one of them?
It was at one time for sure, and I had one environment when I was a customer that it was required, even though F5 advised against doing so. Here's the guide for that, but version is very old. I'd ask your account team to give you a definitive answer on whether support endorses these days.
@Amr_Ali If you would like to have two GTMs in the same location be HA in an active/standby configuration it is possible but it is a big money sink to do such a thing since GTMs themselves are inherently HA when in active/active. You really should have two GTMs run as standalone GTMs in a sync group to keep their DNS configuration synced between them but respond to DNS queries individually. If you had a much high DNS traffic configuration even then you probably wouldn't place the GTMs in active/standby but rather just in a larger group of GTMs so they can all respond to queries and if one fails the rest will continue to respond while you replace the failed GTM. As @JRahm stated his article should walk you through an active/standby setup but you really should have your user or group provide information that 100% supports the setup of active/standby GTMs rather than the normal standalone active/active setup that F5 recommends configuring.
