You're correct that libform is part of ncurses. The problem is simply that ncurses was recently updated to version 6. So /opt/local/bin/gdl needs to be rebuilt with the new ncurses.
You should merely need to run:
sudo port selfupdate
sudo port upgrade outdated
The curious thing, though, is that this change was made in r139469 which predates the release of OS X 10.11 El Capitan so I'm confused as to how you are running into this problem. If selfupdating and upgrading outdated ports doesn't fix it, then certainly running rev-upgrade will:
sudo port rev-upgrade
