I forgot to ask, but why do you think Overwatch is only using the CPU? Are you seeing 0% usage of your GPU in the task manager? If so, this is actually a known bug with Windows 10 where it doesn’t report GPU usage correctly.
Otherwise, your DxDiag looks fine. Your drivers are updated, your monitors are using same resolution, your error section is blank. If it’s just the above issue, you can ignore it. But if your performance is significantly impacted, I recommend opening a ticket with the staff. They can look over other reports you won’t want to share on the public forum.