You don’t need a webhook for that. A regular post-receive hook will work very well.
To create and use such a hook you just have to login on the server where your gitlab is installed and create an ssh key for git user.
sudo -u git ssh-keygen -f /home/git/.ssh/reponame_key
(do not type any passphrase when prompted)
Go to your github account and add the public key (it’s been created as /home/git/ssh/reponame_key.pub) to your project as a deploy key.
have a look at https://help.github.com/articles/managing-deploy-keys if you need help with that.
Once that is done, you just have to configure the connection between your git server and github’s:
add an alias to git user’s ssh configuration (add following lines to /home/git/.ssh/config – create it if it’s not present)
Host reponame
IdentityFile /home/git/.ssh/reponame_key
HostName github.com
User git
Now add the new remote (using the alias you just created) to your repository:
cd /home/git/repositories/namespace/reponame.git
git remote add --mirror github reponame:youruser/reponame.git
Now that everything is in place you’ll have to create the actual hook:
cd /home/git/repositories/namespace/reponame.git/hooks
echo "exec git push --quiet github &" >> post-receive
chmod 755 post-receive
The lastcommand is very important because git will check if a hook is executable before running it.
That’s it!
(Replace reponame, namespace and youruser according to your real accounts and enjoy).
Last note: if you want your name andavatar near commits on github, make sure that the email address you are using on gitlab is one of the addresses inked to your github account as well. You’ll see your gitlab username otherwise.