It looks like Gutenprint is in the repository (and thus free software,
hopefully).
That said though the best thing to do is pay attention to the hardware your
looking to buy before you buy it. That way you don't end up (as its extremely
easy to do) with hostile hardware.
While HP (in the printer arena) has one of the best selections of clearly
free software friendly printers even they aren't perfect. There are many many
HP printers dependent on non-free code. HP does have some pretty good
documentation though which can be very useful in narrowing down what is and
is not free software friendly. There are some other issues related to the
drivers actually bundled with Trisquel not yet having support for whats
currently being sold, and the HP drivers having some non-free pieces which
can also make it difficult to distinguish whats good and whats not of the
current models readily available....
