widely? depends on your viewpoint, but there are quite few consumers (look at the right column) https://archlinux.org/packages/core/x86_64/gnutls/

How is wolfSSL/SSH? I have come across them a couple of times, but not having anyone/anything ”famous” behind them when compared to GNUtls or mbedtls always made me somewhat wary.

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!

One lesser known common user of GnuTLS is GNOME. Pretty much the entire GNOME ecosystem uses it.

One probably important project that uses it is OpenConnect, as the VPN servers would probably be listening on a port reachable on the open internet.