It is sometimes called 'lead grey' which isn't that helpful as lead itself comes in all sorts of shades. When wagons were painted in GC the shade was mixed from black and white pigments in a ratio, if I recall correctly, of nine to one. However, for starters, the paints used did not resemble modern paints and secondly the mixing was not done with any great precision but by individual foremen on site. As the 'sites' were many and various it is highly unlikely that the same shade was achieved every day in every place. (It's worth remembering that many GC wagons were built by contractors, and some were not owned but merely hired. This further reduces the chance of any standardisation.)
If you look at black and white photos you will often see two GC wagons standing together, one looking almost black and the other almost white. To some extent this may have been the effect of 'weathering' - it must be recalled that the GC ran through many areas of intense industrial pollution - but knowing which wagon is the more 'weathered' is not always straightforward to establish. Even the condition of the lettering is not always a guide - at this period many railways used a special white paint for lettering that was particularly resistant to wear.
There is also the question of what looks right on a model. I am no expert in this field but I believe it's generally accepted that to look right a model should be painted in a slighter lighter shade than is strictly 'correct'.
So what is the answer? I don't believe there is a correct one. For many years I used Airfix M3 for GC wagons, but now feel it looks a tad too blue. But really that's just a personal taste. I doubt it's 'wrong'. Or that any other dark to mid grey would be 'wrong'. Comments on the matter are very welcome.