bug#58508: gtg package (Getting Things GNOME) doesn't run

[Pkill9]

This is the error I get when running:

guix environment --ad-hoc gtg -- gtg

```
Traceback (most recent call last):
  File "/gnu/store/0x46vnn6nk10dmkjvg9jmzqx65pmjs4r-gtg-0.6/bin/.gtg-real", 
line 76, in <module>
    gi.require_version('GtkSource', '4') 
  File 
"/gnu/store/vbms7iz53dpdz5iz8ik2blr77w17imva-python-pygobject-3.40.1/lib/python3.9/site-packages/gi/__init__.py",
 line 126, in require_version
    raise ValueError('Namespace %s not available' % namespace)
ValueError: Namespace GtkSource not available

```