In order to use the Account Server API, you need to register a client on the Account Server.
You can register a client using ttnctl:
$ ttnctl clients request my-gateway-editor "Client used to consult and edit gateway information" --uri "https://mygatewayclient.org/oauth/callback" --scope "profile,gateways" --grants "authorization_code,refresh_token"
Executing this command will send a request to the Things Network Core Team whether to accept or decline your OAuth client. Before sending a request, make sure that the URI is a valid OAuth callback URI, and that the description describes accurately the purpose of your client.
Shortly after requesting your client with ttnctl, you will receive an email with your request result.
If your request was accepted, then you can list your clients and get your client_secret:
$ ttnctl clients list
See $ ttnctl clients --help for additional information.