I mean, XP Pro is my favorite OS ever, so I understand your sentiment, but I've had to move on a long time ago, because it's just outdated and it's bascially not supported in the industry anymore. I went from XP Pro to Max OS and Linux, now I'm on Windows 10 and it doesn't seem all that bad.
Horses for courses - you may like Win10 with all the choice it offers in Window themes (any colour you want as long as its white!), effectively compulsory spyware (you can disable user telemetry but some updates will silently re-enable it), on-screen advertising ("Use Edge now - use Edge NOW!!!"), watching those swirling circles for 10-15 seconds whenever you do something significant like launch a program, The Interface Formerly Known As Metro, Anniversary and Creator's updates breaking some of your programs, compulsory updates BSODing your system...
...but I'd rather stick with an OS where I can control everything that happens (thanks more to 3rd party security software that wouldn't be able to work in the presence of Kernel Patchguard), that I can customise (again, thanks to 3rd party XPLite and nLite - NTLite may now offer similar facilities on Win10 but requires online activation making it a no-go for me) and which I can install and configure completely off-line if I choose (again, thanks to 3rd party WPAKill disabling Windows Activation).
Seriously, I'd suggest Linux may be a better choice - Mac OS has good design features but Apple are the masters in user control freakery (when a project developed software allowing OS9 users to skin their desktop, Apple tried to shut them down
while MS supported its Windows equivalent). Linux's main failing is the lack of application-level firewall software (by which I mean firewalls that can control network traffic on a per-process basis, e.g. allowing Konqueror to access ports 80 and 443 but blocking Firefox or any other software from doing so) but I suspect you may not consider that a make-or-break issue.
And again, why should GOG be expected to test on Windows Vista, XP, and older when these are unsupported OSes?
For the same reason that GOG are supporting games that have been done and dusted years or decades ago. Because people want them to and are willing to pay for that service. I have over 700 games bought when XP was still an officially supported OS and if any stop working due to "updates", I expect (reasonably, I feel) GOG (or the publisher, for those in active development) to fix them.
Now getting back on-topic, the game in question is not Vista+ or DirectX10+ but dates from Win 9x and seems very likely to run unaltered under WinXP, with far less demanding system specifications than listed. So the original question seems pertinent - why exaggerate the specs and artificially restrict the OS support (and thereby the customer base)?