Upgrading gir1.2-glib-2.0 helped that error and gajim starts. I found the full list of requirements in /usr/lib/python3/dist-packages/gajim/main.py :
_MIN_NBXMPP_VER = '4.5.3' _MIN_GTK_VER = '3.24.30' _MIN_CAIRO_VER = '1.16.0' _MIN_PYGOBJECT_VER = '3.42.0' _MIN_GLIB_VER = '2.66.0' _MIN_PANGO_VER = '1.50.0' _MIN_SQLITE_VER = '3.33.0'