The gyroscope is pointing towards the centre of gravity, much like a compass points to the magnetic north pole. If you followed the equator while holding a compass, it will point in the same direction no matter at which point you are. Gyroscopes are much like compasses just in 3D space.

So if you imagine following the curve of the earth as following the equator, but the gyroscope points towards the centre of the earth (aka the centre of gravity) so it will always point in the same direction. This is also because the plane doesn’t need to tilt it’s nose downwards as it is pulled down by gravity therefore maintaining a constant altitude.

If the earth was indeed flat (it isn’t) the gyroscope would be attracted to the centre of mass/centre of your “disc”. This would mean that every gyroscope on earth would be pointing towards the arctic (or whichever lie you believe to be the centre), even during a level flight on a plane, or underground. We both know this is not true.

Moreover, I’m sure you’d know that gravity would pull the plane towards the centre, meaning that planes would have to compensate for the huge gravitational differences by flying almost sideways… and falling out of the sky.

Not only that, as gravity would be pulling everything towards the centre, everything would be diagonal the further you get from it. I’m pretty sure that the trees in South America don’t grow sideways. Finally, there would be no possibility for tectonic movement due to the lack of convection currents so there wouldn’t be a single mountain, volcano or earthquake on our desolate planet. Oh yeah, i forgot to mention that the magnetic field wouldn’t exist because the molten iron and nickel would just “drop” into outer space so the sun would have cooked any attempt at life before it got the chance to say “Oh no I’m banned from this subreddit”.