Create TXT Record on GTM / DNS

You can't do with GTM, but you can with BIND. You could use the ZoneRunner to create the record via GUI for BIND.

Here is the information about the supported record types:

https://support.f5.com/kb/en-us/products/big-ip-dns/releasenotes/product/relnote-dns-lc-12-0-0.htmlrn_new

"Additional Record Support

This release adds Global Server Load Balancing (GSLB) support for additional resource record types. BIG-IP DNS now supports Wide IPs of resource record types: MX, SRV, and NAPTR. By adding GSLB support for these new record types, BIG-IP DNS can now load balance queries whose responses contain content other than IP addresses. This allows GSLB for responses that are DNS names to other objects in the DNS hierarchy. This new record support is in addition to the already supported Wide IP record types: A, AAAA and CNAME."