This was missing; the code to parse these numbers was there, but the switch didn't have cases for these types.
https://bugzilla.gnome.org/show_bug.cgi?id=602443
23fcc5e GtkBuilder: Support parsing int64/uin64 values
gtk/gtkbuilder.c | 15 +++++++++++----
1 file changed, 11 insertions(+), 4 deletions(-)
Upstream: git.gnome.org