I was facing the same 403 error . By updating /etc/apache2/sites-enabled/gnocchi.conf and restart apache2, I was able to access gnocchi.

I inserted the following Directory directive before closing VirtualHost directive.

<Directory />
    Options FollowSymLinks
    AllowOverride None
    <IfVersion >= 2.4>
        Require all granted
    </IfVersion>
    <IfVersion < 2.4>
        Order allow,deny
        Allow from all
    </IfVersion>
</Directory>

I am on devstack ocata

# gnocchi metric list
The request you have made requires authentication. (HTTP 401)

then i try noauth

# vim /etc/gnocchi/gnocchi.conf

[api]
auth_mode = noauth
#auth_mode = keystone

then use this env，the GNOCCHI_USER_ID、GNOCCHI_PROJECT_ID and GNOCCHI_ENDPOINT come from /etc/gnocchi/gnocchi.conf

export OS_AUTH_TYPE=gnocchi-noauth
export GNOCCHI_USER_ID=d3c5a671-30be-4f3f-892b-184464fcfdd0
export GNOCCHI_PROJECT_ID=4c718289-51d7-4f3e-b846-ec0875572459
export GNOCCHI_ENDPOINT=http://192.168.3.222:8041

It work,no error