I've recently updated the iRules wiki to reflect the iRules commands that have been implemented on GTM, here for your reference:

iRules Reference: GTM

It lists the commands and events specific to GTM, and also those that are shared with LTM. A few notable additions include:

• cname - Causes the specified name to be sent as a CNAME response.
• member_priority - Returns the priority of the specified pool member.
• whereis - Returns geographical information on an IP address.
• wideip - Returns information about the wideIP upon which the request arrived.

Stay tuned for future releases with more additions and enhancements on the GTM iRules front.