There is always 1 frame pause between each loop.
Few days ago i made a topic on this, and the pause got shorter, but even with a number as small as 1 frame it makes the seamless loop impossible.
I’m pretty sure its just a bug with Gdevelop sound system, the easiest solution to fix it is to just make the song loop 1 frame faster than it normally does.
Such a small thing, but it would make the games feel a lot better.