From looking at my currently installed packages, there are quite a few that depend on gnutls: ffmpeg, gnupg, libcups, vlc, and wget look like the ones that would be most well known. It's possible that they only use it as an option, but normally I'd expect it to only be an optional dependency if that were the case. I haven't looked at any of their codebases though, so no idea what they use it for!