I have no experience with using TTF fonts in GD, but I can imagine something like this to happen when you chose a non-existing font or a font that does not have the characters you want to print out.

It even happens in M$ Word sometimes when you load a document created on another PC where the creator selected a font that does not exist on your system. Even then you get those 'small boxes' you refer to...

Jouke Visser, Perl 'Adept'