What problem you try to solve with GANs? Your RAW HDR photos are really taken by a sensor with RGB mask filtering out whatever is not visible spectrum. Scientific or indistrial imaging mostly use monochromatic information which is later colorized for human eyes.

Even if you get millions of RAW photos they will come from different sensors with different properties (noise, sensitivity, mosaic, orientation...) and you need to to normalize them all. sRGB is most common normalization target for which all prosumer cameras are optimized for.

The format is not really important here as it is usually array of pixels. You need more bits and resolution? Fine, but you also get more useless noise.