def gigabytes(bits=None, bytes=None, kilobytes=None, megabytes=None,
    terabytes=None, petabytes=None):
    """Usage: Convert to gigabytes. Example: gigabytes(terabytes=2)"""
    if bits is not None:
        return bits / 8000000000.0
    elif bytes is not None:
        return bytes / 1000000000.0
    elif kilobytes is not None:
        return kilobytes / 1000000.0
    elif megabytes is not None:
        return megabytes / 1000
    elif terabytes is not None:
        return terabytes * 1000
    elif petabytes is not None:
        return petabytes * 1000000.0
    else:
        raise Exception(
            'You must specify one value. Example: bits, bytes, kilobytes, megabytes, terabytes, petabytes'
            )
