Hi Wheelslip and welcome onboard!
In terms of RTR industrial stock there isn't much I'm afraid. If you went forward into the 50's you'd have plenty of IRM Bulleid wagons to choose from, and you would be able to pick up one of their green or grey A class locos second hand.
For wagons from the GSR period, you'd need to look to kits from the likes of Studio Scale Models (see their GSWR and MGWR convertible wagons) and Provincial Models (they've a few brake vans alongside some GNR wagons). @leslie10646 is the man behind Provincial Models and his kits are easy to put together - especially if you ever tried the likes of an airfix kit.
For a locomotive - this is more challenging: you're looking at kits too or repaints of UK outline. IRM do have an 800 in GSR livery on the way, but she would have been doing top-line passenger work in the GSR period.
Edit: OO Works did a GSR 101 (J15) class that would work well if you could find one second hand, but they are hand-built and not cheap!