I'm no expert diagnosing optics issues and it's hard to make conclusions based on a single corner crop. But that said, I did own a GT71 and researched it quite thoroughly. So based on that as a starting point for you...
If your stars look good in the center and your extreme corner stars look uniformly bad, It may still may be a back focus issue. The GT71 can be very sensitive to back focus errors to the tune of .5 mm or less. It may also be your specific scope may not be up to a 4/3 sensor and have well corrected stars all the way to the corners. Acceptable star shape is subjective, but to summarize what I have read on the GT71 (FWIW)... It is best suited to a 1" sensor and most people have no issues and are very happy with it. Many, but not all have had good luck with a 4/3 sensor, though in some cases you may need to crop the extreme corners. A lucky, much smaller group have acceptable results with a APS-C sensor. I was not one of those. In my case, by the time I cropped, I was down to the smaller sensor size FOV which defeated the wide field I wanted.
If you have good (or better) corners mixed with bad once your BF is dialed in, you may have tilt in your image train.
And of course, a mixture of all the above is possible.