Since a few users notified me that the ID of my songs doesn't seem to work in GD, I took the time to find a solition to the issue. Sadly, I couldn't find anything that could explain why they're not working.
Except for the fact that the compatibility between Newgrounds and GD is rather poor. But there's nothing I can do about it. 
I tried contacting the developer of GD directly, but my e-mail received no response. I can only guess that it'll be resolved in one of the future patches, but seeing that GD has been around for a while and the issue seems to repeat itself every time, maybe it's just out of developer's reach.

There is also another possibility. 
I do not allow direct downloads, modifications, nor use of the songs without my permission. I don't see how it could affect whether it's playable in GD or not, but I guess it's possible.
But, even if that was the case, I will not change the way I'm uploading my music. Sorry (not sorry).