gpt recover: unable to open device '/dev/disk2': Resource busy
By default the gpt command requires a exclusive lock on a drive before preforming an write operation to the drive. This can not happen, when a volume residing on the drive is mounted. With respect to disk2, the typical solution would be enter the following command, before entering the gpt command.

diskutil unmountdisk disk2


The drive is identified as disk2. The volume containing user data on this drive is identified as disk2s2. To mount this volume as readonly, first the volume has to be unmounted, then mounted readonly. The commands are given below.

diskutil unmount disk2s2
diskutil mount readonly disk2s2 


I should note that the volume you were trying to access appeared to be mounted, because you were getting an error message when trying to use the gpt command. The command give below will output text, if the volume is mounted.

mount | grep disk2s2


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