Tim, according to the well known author, Piers Connor, the only drawings of Gate Stock that have survived are some very basic GA's (weight diagrams) of the American built cars for the Baker Street & Waterloo Railway. I rather imagine that if he couldn't find any others, then it is fairly safe to assume that there are none out there?!
He has published his own "tidied up" version of that ACF Co., general arrangement for the LURS.
The GNP&B used mainly Hungarian built cars, so rather than just send you a link for a diagram that could be misleading, I have done a bit of research and ended up tracing out a new GA for you.
Apologies for the slightly crabby drawing - and that there is only one half of it so far: I can only scan A4 here at home, and had to work within that size and format. I hadn't realised just how terribly, terribly out of practice I am with a jolly old fashioned Rapidograph as well!
Being confident that Piers has done the best possible job, I used his drawing as the starting point, and by using my trusty "proportional dividers" in conventional fashion, plus an additional new trick of dropping and manoeuvring various selected photo's over each other on the computer screen, I hope and that my effort is close enough for the task?
I know the above is not an exact science by any means, and that great care has to be taken to allow for image distortion and that dreaded thing called "perspective"!
The drawing of the other half will show the saloon side curves, and differing detail of the gates, frames, bulkhead and awning on these cars.
Hope this will be useful?
Pete.