wondering if that's because the piano plays consistently and thus, when there's a break, it's more noticeable.
you pretty much guessed the answer. in order to have a seemless transition you need to cut the audio at the "zero-cross" line, where it's neither peaking above nor below the zero amplitude line. GB isn't the best thing for editing audio as it doesn't offer the depth of zoom a dedicated audio editor will give you:
(Let the page FULLY load. The link to your answer is at the top of your screen)
that said you're also trying to loop your audio the hard way. once you have a perfect loop in the timeLine, simply grab the very top right edge of the region and drag to the right to loop it as far as you wish.