But the website archives show that version 3 and 4 were under the GPL 2 or 3. Source: ftp://ftp.soest.hawaii.edu/gmt/legacy/
I'm not entirely sure how to handle mentioning GMT licensing. I guess it would depend on which versions are supported for compiling against. As it is, though, the manual cannot be considered correct.
Right now, the manual mentions in multiple places that GMT is public domain. (Github search) However, the GMT webpage lists the license as LGPL3+.
From what I was able to find, GMT versions 1 and 2 were indeed Public Domain. Source: https://www.soest.hawaii.edu/pwessel/papers/1991/EOS_91/eos_91.html
But the website archives show that version 3 and 4 were under the GPL 2 or 3. Source: ftp://ftp.soest.hawaii.edu/gmt/legacy/
I'm not entirely sure how to handle mentioning GMT licensing. I guess it would depend on which versions are supported for compiling against. As it is, though, the manual cannot be considered correct.