I unlocked my phone with Sunshine and when requesting the offer via the online support chat facility I was told after giving them my IMEI "I've checked on the system, and I can see you've modified your phone". Kinda worries me they could tell that remotely. I've never been near the official bootloader unlock.
Anyway - I relocked my bootloader and managed to successfully claim the space just via the App!
But if you have rooted your phone, then you had to have unlocked the bootloader. And if you want the 100GB Google Drive storage, then you need to set the bootloader back to ***LOCKED*** not ***RELOCKED***. If you simply do a fastboot oem lock, you will only get a ***RELOCKED*** status.
Plus, if you attain S-Off after unlocking your bootloader following HTCDev, it is very possible you will have a
*** TAMPERED ***
banner on your hboot screen. This must be removed as well.
To Lock your bootloader properly and to remove the TAMPERED Banner, you need to get S-Off, if you don't already have it. To get S-off, you need to run Sunshine (strongly recommended for its convenience).
I did a Guide to reverting your phone to the point necessary to redeem the Google Drive offer here.