the only difference between the two tests are that the city test is for diagnosis only and the cad at gatwick is the certified test, the protocalls for gatwick are that if you score a boarderline result ie deutuan being pass of 6 units, and you get 6.5 when they will ask you to take the test maybe 3 times to get 3 more scores and then average out the result, if you have passed at city then yes gatwick should be giving you certification, and that should be it, but we all know how accurate and fair the caa are over this
and if anything they should be offering the cad at gatwick for free is they have results from city, and as you might be aware city designed the test and spend nearly 3 million on it.