It seems that many Windows Insiders are getting Windows Phone 8.1 GDR2 before getting Windows 10 for Phones Build 10051. According to Gabe Aul this is normal process. Surprisingly devices like Lumia 930 that are not supported for this Windows 10 for Phones build are also getting GDR2 though not the Windows 10 for Phones subsequently. But if your device is supported you need to go to Windows Insider app and reselect “Insider Fast” to get the Windows 10 for Phones build 10051.
@flobo09 Normal to be upgraded to Update 2. Run Windows Insider again and select Fast ring again.
— Gabriel Aul (@GabeAul) April 10, 2015
In case you are hitting issues with the update process, here are some of the official error threads.