score:5
It doesn't appear that there's any report about Gutenberg not being right-handed.
I tried several sites reporting famous left-handers, and couldn't find Gutenberg on anyone's list (not even Steve Gutenberg). The most extensive seems to be kept by the University of Indiana (although I don't know how well-curated it is. It appears to take public submissions, which is scary).
As one of this site's (many) resident left-handers, I feel compelled to make a few points here: