Is het veilig? Bevat het adware of spyware?

GDM contains no adware, spyware, bundled offers or tracking. Its whole source code is public, so anyone can check what it does, and every release is built from that code by GitHub's servers, where the build can be inspected. The helper programs it can fetch for videos are verified before they run: yt-dlp against its developers' signing key, ffmpeg against its published checksums. Download GDM only from this site or the project's GitHub releases.