Install go-gcm:
$ go get github.com/google/go-gcm
$ go install github.com/google/go-gcm
$ go run server.go -testData testdata/clients.json \
-apiKey <your_server_api_key> -senderId <your_sender_id>
You can get your Server API Key and Sender ID for your project from its Download and install configuration page at https://developers.google.com/mobile/add.