413 is a standard HTTP code. Take a look at Wikipedia it says that :
413 Request Entity Too Large
The request is larger than the server is willing or able to process.
It means what it means, you are making a very big request, maybe a long text or json, or whatever, reduce its size...
According to GCM doc limitations are like so (I think you use HTTP):
HTTP: Downstream only, cloud-to-device up to 4KB of data.