How do I get rid of the margins when printing photos?

Hi,

I'm using Gthumb on Maverick with an HP Photosmart C6380.

In the past (pre-Maverick) we had no problems printing photos on this. Now, when I set up to print on 6"x4" card, it wants to put a big margin on the bottom of the photo page. The printer can print right to the edges (and this is how it is set in the printer properties) and there doesn't seem to be anywhere in gthumb where I can change the margins. The result is that I can only print small photos, as it scales to fit what is left after the margins.

Other programs (shotwell/fspot) can print right to the edges on the same machine.

How can I change the margins so that I can print a full-size photo? Thanks!