For the 10th odd time I've attempted to complete this game. Almost EVERY time all my relevant save files start loading with this bug. This time round a cultist throws 2 frags, there's a load of smoke, a notice able slowdown fo the engine and the CTD. I managed to re save about 2.5 seconds in after the frag throw, but now I get an extra 5 seconds and then it still CTD's.

Has anyone manage to sort out or come across any bugs like this? I've been trying to finish this game for over 20 years now and it's been fighting me EVERY time. I still get constant CTD's or locked black screens after loading new missions. Sometimes takes 2 or three end tasks and re-launch to get in. Getting a touch frustrated with this game now!

haven't played much missions in the campaign (i haven't re-finished the game since i was on window xp ) , mostly using the skirmish (random scenario) so i haven't yet run into this save crash issue for now.

But something you can try in case it's crashing during the chaos cultist turn :
https://www.pcgamingwiki.com/wiki/Warhammer_40,000:_Chaos_Gate#Fix_for_crash_to_desktop_on_Chaos_turn

ignore the first part
"Go to line 201 and change PlazPistHit.wav to PlazpistHit.wav"
As it's already modified in the current gog version of the game, but you can try the other edits (but back that WeapDef.dat file before just in case so you can revert your changes easily)

I've already run through all three options in there (and most of the other ones floating about on this forum) and no dice. Thanks for the suggestion though. I think that guide was to fix the old crash on the first mission when the cultist fires the las pistol for the first time and it used to insta crash.

If nothing works, you can try another dxwrapper than the one gog is using, by example in case the crash is related to something wrong with it (the smoke heavy slowdown is suspicious) , one people have been often using :
https://github.com/elishacloud/dxwrapper/releases/latest

Oh nice, I haven't seen that one before. I'll have a crack and see if that sorts it. Thanks so much!

OK yup, had a play, no dice. Altough I honestly don't fully understand what I'm doing with that ini file. Can't even get the game into the menu.

Don't mess with the game's *.ini files. There are a few parameters you could experiment with to modify animation delays, but most of them won't help with major issues and will only increase the odds of unexpected seemingly random crashes. The first step of any troubleshooting method with older games always implies that you're starting with a fresh non modified version of the game. Make sure you didn't alter it in any way, including your past saved games.
You could be facing rare instabilities due to specific hardware and (mostly compagny-based) software combinations, which might be solved by disabling graphical options - this sort of patchwork -, but don't bet on that...
Sanc's link to PCgamingwiki provides several solutions you could try, but don't limitate yourself to the part he mentioned : most of the time, in my experience, crashes on modern OSes are more likely to be solved by manipulations described in the Issues fixed section.

What you're experimenting is usually tied to DirectX *.dll calls poorly handled by Windows 10+, hence the recent inclusion of GOG DX1-7 configurable wrapper to this release. By tweaking a bit its options, you should be able to fix your problem. If not, indeed, Elisha's DxWrapper could help, but only if you fiddle with its Troubleshooting parameters and try to understand Chaos Gate's specific needs, since the standard default ones apparently don't help many people on this forum.
I wrote "apparently" because I never ran into any of those issues, but that's the benefit of using older versions of Windows. Besides, an easier way of insuring you won't run into any sort of trouble lies in Hyper-V or WMware, to emulate such an older version and install your game on the OSes it was intended for in the first place. Those are efficient ways of avoiding those troubles all together. But, of course, one generally buys Gog's version precisely because he'd rather enjoy an old gem on a more modern rig without having to deal with emulation software by himself... I can't help you more unfortunately.