Hi Amr_Ali , each LTM should be added individually. The iQuery communication that will be established is done with all devices regardless of HA status, so the floating IP doesn't matter from GTM standpoint.
Amr_Ali It should be a matter of ensuring the GTM hardware has the LTM license active and provisioned which will require a reboot of the GTM. Typically it's recommended that the GTMs have their own IP that they accept DNS queries on and not share that with the LTM portion of the configuration other than the management IP itself. You would have HA configured for LTM but only have the sync group for GTMs configured so that the GTMs respond independetly of each other and the LTMs work as physical hardware backups for the other. It is not generally recommended that you combine GTM and LTMs on one platform but it can be done. The reason for this is because the GTMs tend to sit on your edge and the LTMs tend to sit in a higher security area but that doesn't mean you can't do it.
Without regards to the fact that they are combined into an Active/Standby HA Pair ... can you define the two separately, each one with its own non-floating self-IP ... then define the two of them as aProber Pool -- choosing to load balance between the two LTMs via the Global Availability load balancing method.
thanks for your support, as below screenshot after adding two LTM with their own external self IP, the LTM that is on standby mode is still down on GTM, and the LTM that is on active mode be active server on GTM