Hey, I have a lot of experience with GCM, using many different servers (PHP, Java, C#). I have used GCM in 4 or 5 android apps, so I know what I'm doing and I can debug your issue and get your notifications going again. I will do the work with a tiny deposit, then nothing till you see your notifications working.
Please message me if you would like me to look at it (free) and give a more informed quote, it may be a simple task that won't cost you much.
Best regards Warrick, I look forward to working with you.