I'm migrating my app from C2DM to GCM but the server on which i'll be sending the registration id will still be entertaining C2DM ids. So my question is what can I do from my app so that server can identify that this is GCM id. Google recommends to send a new bit along with registration id. How can I do that. Please give an exmaple.
Here's what you need to do if you're not using AppEngine.
In your server, you should already have a table that stores all the registration id (reg_id). You need to add another column to this table, say 'is_gcm_reg_id'. You would default all existing rows to '0' for this column since they are currently all C2DM reg_ids. You could also just create a new table to store the GCM reg_ids, if you like.
Then in your GCM enabled application, you just need to let your server know the reg_id is a GCM reg_id. You didn't mention how you upload the reg_id to your server (Web Service call? Simple POST request?). If using Web Service call, just create a new method that will be used exclusively by your new GCM-enabled app. If using POST request, just add another key value pair like 'gcm=true' and your server should look for this kvp.
Finally, you should have all the pieces needed to know the reg_id is of GCM, and update the 'is_gcm_reg_id' column field (or a new GCM table) accordingly.