It is necessary to distribute redmine plugins under the GPL License? As i understand GPL licence - answer is yes. Cause redmine and plugins used one space. It similar like wordpress or joomla plugins But some guys ignore this requirements of GPL example Is it necessary to distribute redmine plugins at all? No. Gpl allow to develop gpl code without distribution. But since you distribute some product you should distibute them source code.
Full article: http://www.redmine.org/boards/4/topics/27405