The link to the "Upstream URL" is dead, but there is also this other page which still works:
https://sourceforge.net/projects/gtkpod/
As other comments mentioned, it seems pretty dead. The wiki on that page is useless, but there is a mailing list that might be helpful. gtkpod also has a useful manual under "Help" in the menu bar.
I wanted to use gtkpod to sync some playlists to an old iPod, since some of the feature seemed good, but in the end I couldn't get it working and decided to use clementine instead which "just works".
Pinned Comments
bakerty commented on 2020-09-05 00:21 (UTC)
Made some small code tweaks which fixed the compile problems for me. Source is linked here.