Is there an article or YT video that explains all differences between Retail PC, PS2 and GOG versions?
PS2 is the original version. Xbox and PC were ports so I'll tell you what changed in the PC port instead. I have very little knowledge of the Xbox port but the only notable difference is that the noise effect is heavily reduced on Xbox.
PC/GOG are nearly identical except GOG supports Widescreen and fixed the 15fps cutscenes.
PC is missing the noise effect in high resolutions, the default color settings are also off you have to re-adjust them to make them look like the PS2 version, the real time shadows are broken (no smoothness) but they're broken in every Silent Hill game on PC, the original PC version had cutscenes at 15fps but this is now fixed on GOG, the FMVs are stretched on PC but it can be fixed with a mod, this version also accidently disabled 5 or 6 hauntings that can appear on PS2/Xbox (this could be fixed if someone cared enough to make a mod to re-enable them) and controller support is still janky without mods.
Overall, it's an okay re-release and most things can be fixed with mods but could be a lot better.