Is it advisable to have GTM in HA pair ?

This configuration is not generally the recommend practice unless you have additional modules (such as LTM, ASM) on the same box. Most installations I have worked on have a single GTM in each DC which in turn have iQuery connections to local and remote LTMs.

This tends to be because GTM sits on the internet edge and keeps your DNS infrastructure separate from your web front end, adding a layer of security