THIS PLUGIN IS DEPRECATED IN FAVOR OF LOGIN-OAUTH2 - https://github.com/trilbymedia/grav-plugin-login-oauth2
Grav Login Plugin OAuth Add-On
The Grav Login Plugin OAuth Add-On for Grav adds the ability for users to log in using Social sites.
Currently Available Providers:
- Github
Installation
The login-oauth plugin actually requires the help of the login plugin.
Both are available via GPM, and because the plugin has dependencies you just need to proceed and install the login-oauth plugin, and agree when prompted to install the others:
$ bin/gpm install login-oauth
Usage
Add a login-protected page, then make sure you fill the "Route" in the Login plugin settings. Add this route to the callback url required by the OAuth application on the service desired. Example: http://yoursite.com/login.
Note: OAuth has not been tested with Grav's multilang feature! Due to this, certain OAuth providers may not function properly on multilang sites
IMPORTANT:
localhostmay NOT be used for callback and allowed URLs when creating OAuth provider applications due to certificate verification issues. Some services allow other URLs and it may be possible to add custom domains pointing to 127.0.0.1 in your hosts file and point applications there. GitHub and Twitter are tested to work on localhost too, if it does not work you can use a tunnel like ngrok to test locally
Visit https://developers.facebook.com/quickstarts/?platform=web and create an app name then click Create New Facebook App ID.
Choose a category most similar to your business then click Create App ID.
Scroll down on the next screen to the section titled Tell us about your website. Input a URL for the site (no need to include the protocol). Click Next
Click Skip Quick Start Copy the App ID and App Secret into the plugin configuration under Facebook.
On the left hand side click Settings
In the Basic tab add your domain into the App Domains section as well as enter a contact email (required for facebook developer program). In the Product Settings menu click "Facebook Login". Scroll down to the Client OAuth Settings Make sure that Client OAuth Login is enabled as well as Web OAuth Login is enabled. In the Valid OAuth redirect URIs section add the routes of all pages that are protected by login. This includes the domains. EX: http://getgrav.org/, http://getgrav.org/login, http://getgrav.org/en/login, and http://getgrav.org/protected/page/route
Github
Visit Github's Developer Applications Console and press button Register new application (login if necesarry). 
Fill out the name and the URL (can be anything) and fill in the callback, which must be equal to where your grav site is located, generally just the host, i.e. http://getgrav.org. 
Copy Client ID and client secret into the plugin configuration under Github. 
Be sure to change Github.enabled to true
Visit the Google Developers Console (sign in with a google account, preferably your businesses gmail).
Select Create Project and give the project a name (can be anything). Click Create. 
When it's finished creating in the left hand menu choose Credentials under APIs & Auth (you may need to click APIs & Auth in order to display Credntials). 
Under Add credentials (center of screen) select OAuth 2.0 client ID.
Then select Configure consent screen in the top right corner. 
The only requirement is Product name which should be the name of your website/business (not a url). You may fill in the other options as you want on the consent screen. (The consent screen can also be changed later). 
Then once you save the consent screen select Web application from the radio buttons and fill in the fields. Name being name of product/business. Authorized Javascript origins is the root domain name of the login page (no routes or wildcards) such as http://getgrav.org.
If needed, enter multiple sub domains, creating an entry for each. Authorized redirect URIs include the same Authorized Javascript origins used along with the route to the login page such as http://getgrav.org/login. Click create.
Copy Client ID and client secret into the plugin configuration under Google. 
Be sure to change Google.enabled to true
Login if necessary. Create a new Twitter App , fill out name, application website, choose "Browser" as application type, choose the callback URL like above, default access type can be set to read-only, click on "Register application" and then you should be directed to your new application with the Client ID and secret ready to be copied and pasted into the YAML file.
Go to your Apps section in LinkedIn Developers, and create an application. After that you will get a Client ID and a Client Secret of your app, copy and paste them in the config file or use the admin panel.