After a recent update to nautilus, GTT says:

werewolf:~> gnome-tweak-tool
Traceback (most recent call last):
  File "/usr/bin/gnome-tweak-tool", line 76, in <module>
    MainWindow()
  File "/usr/lib/python2.7/site-packages/gtweak/mainwindow.py", line 44, in __init__
    model)
  File "/usr/lib/python2.7/site-packages/gtweak/tweakview.py", line 40, in __init__
    self._model.load_tweaks()
  File "/usr/lib/python2.7/site-packages/gtweak/tweakmodel.py", line 135, in load_tweaks
    mods = __import__("gtweak.tweaks", globals(), locals(), tweak_files, 0)
  File "/usr/lib/python2.7/site-packages/gtweak/tweaks/tweak_nautilus.py", line 48, in <module>
    GSettingsSwitchTweak("org.gnome.nautilus.desktop", "computer-icon-visible", schema_filename="org.gnome.nautilus.gschema.xml",group_name=TWEAK_GROUP_DESKTOP),
  File "/usr/lib/python2.7/site-packages/gtweak/widgets.py", line 116, in __init__
    _GSettingsTweak.__init__(self, schema_name, key_name, **options)
  File "/usr/lib/python2.7/site-packages/gtweak/widgets.py", line 105, in __init__
    options.get("summary",self.settings.schema_get_summary(key_name)),
  File "/usr/lib/python2.7/site-packages/gtweak/gsettings.py", line 122, in schema_get_summary
    return self._schema._schema[key]["summary"]
KeyError: 'computer-icon-visible'

A workaround is to comment the key on the schema, but a better solution looks like this patch:

http://git.gnome.org/browse/gnome-tweak-tool/commit/?id=a83ddd89364b84d34d70cdfefe694705b0098b2b

Perhaps you could update to latest git till a new version is released.