I installed gnucash with `guix package -i gnucash`. When I attempt to
run it, it fails with the following error message:

**
gnc.gui:ERROR:gnc-icons.c:96:gnc_add_stock_icon_pair: assertion failed: (pixbuf1 && pixbuf2)
fish: 'gnucash' terminated by signal SIGABRT (Abort)

However within an environment created by `guix environment --ad-hoc
gtk+`, gnucash runs fine. So, I'm guessing gtk+ needs to be in gnucash's
propagated-inputs.