Some time ago I mentioned the sad fact that the extremely useful Manvers Main wagon had vanished from the Slater's range.
I am glad to note that Manvers Main transfers and/or pre-lettered kits and RTR kits are now available from Powsides. This is excellent news. The reference is ID0956.
There are of course many other wagons that are suitable for the GCR (or parts of it) available from Powsides. Although for the present the South Yorkshire coalfield is lamentably under-represented, while there are hosts of wagons for obscure railways serving the south and west of England. This probably reflects customer demand, as it certainly doesn't reflect the proportions of prototype wagons.
If you are modelling pre-group, avoid anything intended for 1923 standard wagons. This ought to be obvious, but observation at exhibitions suggests it isn't. If you look at the Powsides online catalogue, the pictures of the various transferred can be 'zoomed'. This does not always, unfortunately, enable the subsidiary lettering to be read. Again (obviously) for pre 1923 any reference to LMS or LNER (or indeed SR) should be avoided. If in doubt, ask Powsides. The converse is not necessarily true, as pre-group designations on wagons remained, in some cases, until BR days!
As ever, I recommend the excellent books by Keith Turton and Bill Hudson on this subject. One thing is certain - if you are modelling the GC at any time up until the 1960s you will need some PO wagons, and if you are modelling pre 1939 you will need (relatively) a lot. After 1939, they did get grubbier, and some were eventually repainted plain grey. Again, do check the published works for details.