Hello, welcome to my thread, feel free to add on to the list.
Here are some possibilities to unlocking the BOB/GRD Helmet.
-Kill all "Golden -Ranger- Elites"
-Find all Datapads (Maybe there's an incrypted 'code'?!)
-Hold/ Press a certain button on all GRD Dolls
-Find "Forerunner Letters" on each of the new Anniversary Maps