[@TK] I tried GrabTube before recommending it to others. Regarding the FFmpeg concern — GrabTube uses FFmpeg as a standalone external binary (called via command line, not linked as a library). This is the same approach used by VLC, HandBrake, and many other apps that ship with FFmpeg.
The FFmpeg builds bundled are from the official BtbN/FFmpeg-Builds repo on GitHub. Since GrabTube calls FFmpeg as a separate process rather than linking it into the application code, this is generally considered compliant under GPL terms.
That said, you make a fair point about including the license text in the installation. I’ve passed the feedback to the developer — adding a copy of the GPL/LGPL license files in the app directory would be good practice for full compliance. Hopefully they address it in the next update.