I want to push notifications to around 50,000 users at a time and about 50 notifications per day, is it a good choice to use GCM in this case? If not can i know which other
I think that GCM is a good choice to use. It's reliable and using it helps to conserve battery and data usage since it piggybacks other Google services. All you need is Android 2.2 or later with the Google services installed, which means no Kindle Fire.
I do not think that GCM would have any problems handling the number of messages or devices that you gave.
If you use it you will still have to write your own server component to handle registrations and message sending. I wrote a blog post that describes how this works.
Some commercial services that handle the server component for you (as well as other things) are AirBop, UrbanAirship, and ClixAp. Parse is a commercial solution that (I believe) does not use GCM. As I noted in the comment above I helped create AirBop