_timezone_gmt(dt, 'short', locale='en')
u'-0700'
>>> get_timezone_gmt(dt, locale='en', width='iso8601_short')
u'-07'

The long format depends on the locale, for example in France the acronym
UTC string is used instead of GMT:

>>> get_timezone_gmt(dt, 'long', locale='fr_FR')
u'UTC-07:00'

.. versionadded:: 0.9

:param datetime: the ``datetime`` object; if `None`, the current date and
                 time in UTC is used
:param width: either "long" or "short" or "iso8601" or "iso8601_short"
:param locale: the `Locale` object, or a locale string. Defaults to the system time locale.
:param return_z: True or False; Function returns indicator "Z"
                 when local time offset is 0
