On a hunch, I removed the b thinking it might have been just a second generation and did a google search for 'thermostat model GT 5000' and came up with GemTech. I am providing a link to their operation notes below so you can see if it's what you have.
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Here is a link that might be useful: GemTech GT 5000 operation notes

There is usually more info on the backside or on the mounting plate if you detach it from the wall.

I should have given the link for the picture of it. That's below.

Here is a link that might be useful: Pictre of GT 5000

Here are the operating instructions for that thermostat..
http://gemtech-thermostats.com/images/GT_5000_operation_guide.pdf

E.W.