enable-app-engine-project
tries to locate sources based on the path(s) where the modules are installed on your local system. Have you installed these modules locally?
As an alternative, you can simply copy the following directories into your App Engine project manually:
SOURCES = [
'gflags',
'gflags_validators',
'httplib2',
'oauth2client',
'oauth2',
'apiclient',
'uritemplate',
]