MacPorts base automatically compresses all ports' manpages after the destroot phase. It is also supposed to handle repointing symlinks to the compressed version as needed. In this specific case, that seems not to have happened, possibly because the symlink in question points to another symlink.
Replying to ballapete:
lrwxr-xr-x 1 root wheel 8 6 Sep 11:59 /opt/local/share/man/man1/dot2gxl.1 -> gv2gxl.1 <====
This symlink was created by the port and now points nowhere because MacPorts base changed gv2gxl.1 to gv2gxl.1.gz.
lrwxr-xr-x 1 root wheel 11 10 Jul 22:04 /opt/local/share/man/man1/dot2gxl.1.gz -> gv2gxl.1.gz <====
Based on the different timestamp and the fact that I don't see this in the package, I assume you created this symlink. This is the symlink MacPorts base should have created.