The old Python gdata service libraries support ClientLogin, AuthSub and OAuth 1.0 authentication. All of which have been deprecated. If you wish to use the OAuth 2.0 Service Account credentials you'll need to hack something together like:
def buildSpreadsheetService():
scope = 'https://spreadsheets.google.com/feeds'
credentials = SignedJwtAssertionCredentials(sa_id, sa_key, scope)
http = httplib2.Http()
http = credentials.authorize(http)
build('drive', 'v2', http=http)
sheets = gdata.spreadsheet.service.SpreadsheetsService()
sheets.additional_headers = {'Authorization': 'Bearer %s' % http.request.credentials.access_token}
return sheets