$ cat /usr/share/glib-2.0/schemas/test.gschema.xml 
<?xml version="1.0" encoding="UTF-8"?>
<schemalist>
  <enum id="test.WrapMode">
    <value value="0" nick="none"/>
    <value value="1" nick="char"/>
    <value value="2" nick="word"/>
    <value value="3" nick="word-char"/>
  </enum>

  <schema path="/apps/test/" id="test.editor">
    <key name="wrap-mode" enum="test.WrapMode">
      <default>'word'</default>
      <aliases>
        <alias value='GTK_WRAP_NONE' target='none'/>
        <alias value='GTK_WRAP_WORD' target='word'/>
        <alias value='GTK_WRAP_CHAR' target='char'/>
      </aliases>
    </key>
  </schema>
</schemalist>

$ gsettings set test.editor wrap-mode \'none\'
$ gsettings set test.editor wrap-mode \'GTK_WRAP_NONE\'

(gsettings set:25795): GLib-GIO-CRITICAL **: g_settings_set_value: assertion `g_settings_range_check (&info, value)' failed
Key wrap-mode is not writable
$ 