GTM is gas time remaining based on current depth and gas consumption rate.
Also strictly my opinion and I certainly would never tell someone how to use their computer, but I would caution using your computer in tech mode while recreation diving. That's a discussion for another thread! I love my perdix in both side mount and back mount!
No mention of "GTM" in the manual. GTR is Gas Time Remaining.
I use my computer in OC Tec mode when tech diving. I don't see any reason to change it just because I'm doing a recreational dive.