shep wrote:If you are using the lpr print spooler, lpr support was removed from gtk3 applications but remain in gtk2.
So that's probably the reason why I was able to print from GTK2 applications but not from GTK3 ones. I thought it was a GTK3 related issue but after reading your post I kept searching and I found out that the printer, a HP M402n, required
at least HPLIP version 3.15.11 while the version in the Debian stable repository is 3.14.6.
I
updated the HPLIP related packages from the jessie-backports repository and after
selecting the dnssd:// connection and
selecting Raw for the Make/Manufacturer as specified in the
Debian Wiki, I was able to get the printer working correctly even from GTK3 applications. The parts I highlighted in bold are important since it's otherwise easy to oversee that part; looking for the HP M402n model in the CUPS list of Make/Manufacturer doesn't display any coincidences, you have to select Raw.
Thanks, marking this topic as solved.