The Handy box shown above will NOT accomodate GFI device.
We always just hung a Romex tail out the building sheathing with a DEEP single gang "Masonry" box w a two screw connector hanging dangling on the tail.
Leave the two screws on the connector slightly loose so cable can be slid in or out.
You do NOT need anything fancier or more expensive.
Mason would exact position and bed into place.
Later, Electrician installs device and uses silicone between gasket and brick.
Done.
Notice the threaded 6/32 holes in link below are INSIDE the confines of the box so the mortar cannot get in the way of the device screws.
https://www.homedepot.com/p/RACO-Si...-3-4-in-Concentric-KO-s-25-Pack-695/203634432
Do NOT use a shallow "Handy box".
Do NOT hang out less than like 15" total tail of Romex.
In OP situation, best to have only ONE cable to the box... mostly so bricklayer does not need to fight an octopus.
The easier you make it for the bricklayer, the better job he will do... hopefully anyway.
Marc