Add the following dependency to your project BuildConfig to enable rest calls using HttpBuilder.

compile ":rest:0.8"

Save your data in the Config and use it in database, config used in this example is given below.

youtube{

    clientId="your_client_id"
    clientSecret="your_client_secret"
    scope ="https://gdata.youtube.com"
    responseType ="code"
    accessType="offline"
    redirectUrl = "http://localhost:8085/youtube/callBack"

    url{
        auth =  "https://accounts.google.com/o/oauth2/auth"
        token = "https://accounts.google.com/o/oauth2/token"
        userInfo = "https://www.googleapis.com/oauth2/v2/userinfo"
        dataApi = "https://www.googleapis.com/youtube/v3"
    }

}

Use this controller to authenticate the user from the google account and use youtube api. You should implement the logic to save the response data i.e. refresh token in database.

class YoutubeController {


    YoutubeAuthService youtubeAuthService


    def auth(){
        response.sendRedirect(youtubeAuthService.createAuthorizeRequest());
    }

    def callBack(String code){
        // User did not rejected authorization of your project
        if(!code)
            redirect(action: 'Error Action') 
        def object = youtubeAuthService.getAccessToken(code)

        // Save your Youtube Tokens to DB
        // or hit data api to get User info using access token and then save
        
        // chain the request to your success handler.
        chain(controller: 'controller', action: 'action')
    }
}

Use this service to get the access token and refresh tokens

package com.ekiras.youtube

import groovyx.net.http.HTTPBuilder
import groovyx.net.http.Method

import static groovyx.net.http.ContentType.URLENC

class YoutubeAuthService {

    def grailsApplication


    String createAuthorizeRequest() {
        String clientId = grailsApplication.config.youtube.clientId;
        String authUrl = grailsApplication.config.youtube.url.auth;
        String redirectUrl = grailsApplication.config.youtube.redirectUrl;
        String scope = grailsApplication.config.youtube.scope;
        String responseType = grailsApplication.config.youtube.responseType;
        String accessType = grailsApplication.config.youtube.accessType;

        String authRequest = authUrl + "?client_id=" + clientId + "&redirect_uri=" + redirectUrl + "&scope=" + scope + "&response_type=" + responseType +  "&access_type=" + accessType
        return authRequest;
    }

    def getAccessToken(String code){

        String clientId = grailsApplication.config.youtube.clientId;
        String clientSecret = grailsApplication.config.youtube.clientSecret;
        String tokenRequestUrl = grailsApplication.config.youtube.url.token;
        String redirectUrl = grailsApplication.config.youtube.redirectUrl;
        String grantType = "authorization_code";

        Map data = ['client_id':clientId,'client_secret':clientSecret,'code':code ,'redirect_uri':redirectUrl,'grant_type':grantType]
        HTTPBuilder httpBuilder = new HTTPBuilder(tokenRequestUrl);

        httpBuilder.request(Method.POST){
            requestContentType = URLENC
            body = data;
            response.success = {resp, reader ->
                // reader contains a access_token,token_type and expires_in
                return reader 
            }
            response.failure = {response, reader ->
                return null
            }
        }
    }

}