Now you are able to make multiple calls on Gmail with the new feature Google just launched. You can either receive or call multiple numbers at a same time without having to drop a call. Previously you could only just make a single call at once. And the feature works on all voice, video or phone calls.
According to the Gmail Blog, when you are on a call with someone and want to make another call you are allowed to do so by setting the previous call on hold. And to switch you should just press the resume button which automatically puts the other call on hold. For receiving multiple calls, you will get an notification as you receive a call and when you get a new call the previous call is put on hold.
This feature has a restriction that you can only call two physical phones at once. However this is a great feature that is going to help a lot of people and since Google provides free calls to US and Canada. This would be much appreciated by the Gmail users.