I am also currently searching for the same exact item (damper motor GA58LB-5-6-46-136). Two of them in fact. I'll let you know if I am able to find anything.
Other details / specs:
"Synchron" round motor
24V 60 Hz 6W 1 RPM

After many hours of frustration searching I found a youtube video (https://www.youtube.com/watch?v=HuuGhuAm1zE) by a company that seemed to be experts in identifying replacement solutions. As I dug further into my system, dismantling and taking the necessary pictures, etc., I came to realize the motor actually was able to engage and run... but the connection to the offset pivot had failed. I could tell this because I was able to move the flat damper actuator arm up and down (to open and close the damper veins) manually. The offset pivot simply spun freely around the motor shaft.

I assume since our motors have identical numerical codes, your setup might be similar. If so, check for the same symptom and if your problem is actually like mine (i.e., motor not actually fried) then I have some good news. This worked like a charm for me and I'm actually repairing my 2nd unit right now. I dabbed 2-part metal epoxy around the edge of the assembly (I used Loctite Epoxy Weld but something like JB Weld I think is same). I let it cure for about 3 hours (24h is full cure). I reassembled and watched in amazement as the damper now works perfectly!!:D:D:D I have pics and will try to get them uploaded tomorrow, assuming this forum supports pics.

As to your question for how to remove the old motor from damper:
Installations may vary but here's what I did...
There is a tiny clip that holds damper actuator arm ( a flat blade) to the offset pivot on the back of the motor assembly. Just carefully pry that off using a small sharp screwdriver.
Then remove three small screws holding the motor assembly to the damper.
And finally, remove the wires (likely 3... I had a red, white, and blue running from the zone control panel) from the damper motor assembly by loosening each connection using a tiny flat screwdriver.
The motor assembly should now be free from the damper.
I had been quite puzzled how to remove the motor itself from the assembly but that became a moot point when I discovered the real problem.

Congratulations on solving your issue! And thanks for such clear pictures and detailed instructions! I do have the exact same model as yours, and I could move damper actuator arm up and down manually and had since left it open to easy the pressure on blower. So it is likely that I have the same problem as yours. However, my damper is installed at the 90 degree duct joint and have a very tight space between the duct and the back of the motor. Not sure if you had the same situation - if so, did you remove the damper from the duct?