2 Answers
It will be "gp2"
I’ve not actually tested this, however per this GitHub issue https://github.com/kubernetes/kubernetes/issues/34549#issuecomment-700115626 there is no validation for two different Storage Classes (SC) set to "(Default)".
There is validation around provisioning a Persistent Volume Claim (PVC) when more than 1 SC is set to "Default". You will not be able to provision a PVC as you will see a forbidden error "forbidden: Internal error occurred: 2 default StorageClasses were found"