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
```
