These models (GP38, GP39 and GP40) all came out at the same time, and as such I'm going to go ahead and assume that they all have the same basic chassis/mechanism. So, to save myself a bit of time (and money), I'm going to review them all here.
These are all very typical 1980s-vintage Samhongsa brass diesels - IE, they look great and they run good (but not great). Also typical is their lack of window glazing, couplers and lighting.
The chassis is all-metal and fairly light (with most of the actual heft being provided by the brass shell and the fuel tank). The motor is an open-sided 5 poler. The 3-piece driveshafts are all-plastic. All wheels are geared and provide pickup (via wheel-wipers). All of the truck gearing is metal. There are no traction tires. Current is transferred from the trucks to the motor via wires (two per truck).
As mentioned above, performance on these models is decent (if unspectacular). Throttle response is smooth, slow-speed creep is fine, the top-end speed is very reasonable, and pick-up is quite good. On the downside, they're fairly noisy. And of course, lacking such niceties as flywheels (and low-friction / wireless pickup), they pale in comparison to more modern diesel models. Overall though, they are respectable runners (particularly given the era in which they were produced).
In 1984, S&R Models (Jacksonville, FL) contracted Hallmark to make a special run of brass GP-38s with high hoods and painted in Southern RR livery. These were the first Southern high hood locomotives ever sold in N scale. This was an exclusive run (50 in total) made solely for S&R Models. Sorry, I don't have any pictures, so send me one! And thanks to Steven Campbell of S&R Models for the preceding factoid.
To remove the shell, unscrew the four screws on the bottom of the chassis (two next to the fuel tank and two next to the opposing truck). The shell should lift up and off at that point.