I am sending GCM notification to device1 from device2 and then device2 sends an acknowledgement to device again using GCM. But when I had the API key as server key then device2 could not send acknowledgement and in log cat device2 showed error "MismatchedSenderID" .
I fixed it by using "browser key" on device2. Here I had to use "browser key" since it was not working with server key . Now My question is : Why it is working with "browser key" not with "server key" ?
Its good if Above solution is working for you ,but according to my opinion ,you should use server key on server side (e.g webservices) ,android key on Android apps .
It should work with "server key" as well as with "browser key " . But in some cases if any one of them is not working then you should try another one .See here