Interesting accident - I hadn't heard of that one. That kind of gyro error is known as apparent wander, and is well known and understood by gyro-driven navigation system designers and users. Gyros, both the traditional moving-part type and the more modern ring laser design, are affected by the rotation of the earth and by an aircraft (or spacecraft, or boat etc) moving around the globe, creating an error known as transport wander. All of these things are well documented, their correction is a fundamental part of the design of numerous navigation systems, and they are of course completely inconsistent with a non-rotating flat earth. I have never seen a credible FET explanation for any of this, probably because there isn't one.