Custom Notification

I am playing GMod with friends. One of my friends goes online, but instead of the standard Steam notification, it says "••••• is now in penis mode." We both find it really funny and want to know how he did it. We look at his profile and notice he is a GMod Workshop creator, so we think maybe you need to upload a mod to unlock that feature. Then, we find out that ••• also has that custom notification.