Is it under GPL, or under GFDL? If the latter, you probably have to include a rant by Richard Stallman about the magic of Free Software on the back of the shirt

Is it under GPL, or under GFDL? If the latter, you probably have to include a rant by Richard Stallman about the magic of Free Software on the back of the shirt (GFDL has clauses to force you to reproduce… well, that).