I'd say it is very common. GPS is one of two systems used for time transfer in the UTC itself meaning this is done routinely. Granted, it is the less accurate system, but the reason they used in here is presumably because it is good enough. Note that their clocks were calibrated by PTB and METAS and checked by movable time transfer. Hence, I think we can be pretty sure that someone would have told them if they were doing something wrong.As I have said before, the application of using GPS to synchronize two distant stations to a nanosecond is not common, and as such I am less confident that the firmware in the unit is bug-free than had the application been more widely used.