I went into MySQLMan and looked at the actual table & field properties. I did a properties view of gfroum_forum and then did a change on the forum_name field. I changed the varchars setting to 75 and was able to add a forum via the gforum admin panel in my test environment with a name that was 65 characters long. It showed it on the main forum page fine but it did chop off the end when I went back into the admin interface to change or modify the forum -- the def files may help that, I haven't done much with those.
So maybe doing the database change itself and the def files together will work.
Sorry if this doesn't help you - just thought I would throw it out there in case it might.
Best of luck.