If you're Gold or Plat status with Marriott, you'll probably get upgraded or you can put in the comments section, request upgrade.
But if not G/P & you want to 'guarantee' the upgrade to exec room or suite, that's the fee.
Grosvenor Square (about 5 minutes away from GH) has a 75GBP+VAT fee for confirmed upgrades I believe.
Cheers.