The project homepage is located at http://gnuplotfortran.sourceforge.net.
You can use the bug reporting interface at sourceforge (see homepage). Or informally inform me via a posting on comp.lang.fortran.
I am certain that more routines can be added to gnuplotfortran (especially the super-canned kind). So, you may use the feature request interface at sourceforge. Or use the fortran newsgroup.
There is a mailing list at sourceforge, but in my experience, there is never a reason to use a mailing list when the Usenet exists. Spamming a thousand people with a letter in their mailbox when you can pose a question non-intrusively on any one of the newsgroups and invite an informed response is one of those "bright" ideas, whose origin is a little obscure to me.
I cannot imagine a circumstance in which you will need to contact me directly via email instead of using the Usenet or any of the other methods suggested above. But for the record, here it is : msc@ieee.org