Your startup is failing before Windows Boot Manager gets loaded, which suggests a problem with your EFI System Partition. After converting to GPT, check the drive to make sure the ESP was created properly: You should see a new, small FAT32 partition (usually 100MB) that contains the Boot Manager files.
You didn't mention how you converted the drive, but if you're using Microsoft's official MBR2GPT tool, the ESP should be created as part of the process. Also, if you have multiple drives connected, make sure there's only one ESP present across all drives.