You'll need to post the registration request for us to help you.
Looking at the
[docs](https://gluu.org/docs/ce/3.0.1/api-guide/openid-connect-api/#registerget)
for this method makes no sense to me.
The [OpenID Connect Client Reg Spec](http://openid.net/specs/openid-connect-registration-1_0.html#ReadRequest) indicates that the request should include a
"Registration Access Token"
I'm going to ask one of the developers about this.