Determine GTM availability with rest

I'm trying to create a status page that can report when user defined LTMs and GTMs aren't configured or functioning correctly.

With iControlRest, I can determine availability for LTMs by inspecting the value of "state", but I'm struggling on the GTM. Using "/mgmt/tm/gtm/pool/poolname/?expandSubcollections=true" provides "enabled" on poolstate and memberstate, but doesn’t actually give the up/down state that the LTM does.

I’d really like to retrieve the value that the F5 UI displays for Availability. i.e. offline (enabled). Has anybody else run into this? Am I missing something obvious here? Any ideas or direction would be greatly appreciated.