So, I don't know how helpful this will be... but wanted to add some perspective. I see a lot of frustration on the internet with getting the games to work properly with different resolutions, etc.

Originally, I bought all these games back in the day (I have the CDs of them) and played them on a Pentium 2 or 3 or something back in the day. All seemed to work well and never ran into any issues. When they showed up on GOG, I bought all three (had to wait a bit for Afterlight till they got the copyright stuff fixed), but consistently ran into issues getting them to work. I suspect it was because I was trying to install them on machines that weren't supported.

I just installed all three on my Lenovo Legion T7 (Core i9 something or other), with a nVidia 2080 something something, and they all run *perfect*. Not only perfect, but the videos and the entire game fully resizes appropriately. I did nothing... I didn't configure anything,... I literally just installed through GoG Galaxy. Best I can figure, these games do not support some graphics modes, in particular... the built in Intel graphics that most "business" laptops have.

I'm making the assumption that it works flawlessly on my newer Lenovo because the nVidia graphics card software is properly adjusting and compensating for the different graphics modes. Again, don't know how helpful that will be other than to say that newer systems WILL work well.

I'll have to add my opinions after playing UFO:Aftermath and UFO:Aftershock for the second time.

Aftermath.
Simply put I had to throw in the towel. Rage quit if you must describe as such. I started on "normal" difficulty, and at the "retrieve the Reticulan crashed ship" part of the game's story it just got I.M.P.O.S.S.I.B.L.E! The frustration was just way beyond my zero tolerance for such shit. I could not drop the difficulty to "easy" as once set, it's SET. I would have had to start a whole new game from scratch on "easy". That I had NO patience left for. The cheats are all disabled in the version GOG are peddling, v1.4. First time I played it I did finished it, I must have played on "easy" that time!
But despite the rather weak (even for 2003) graphics, I did like the game - until it destroyed my nerves and will to perservere with the difficulty.

Aftershock.
WHAT A DISASTERPIECE. In fact the day before that penny dropped I'd given it a 2-star review. 2 stars too many. But you can't even EDIT your own reviews here on GOG - and never will. We've been screaming at GOG to overhaul the review systems for years. They have ignored us completely.
First off, the BUGS that will assail you from square one to final square make Aftershock a truly harrowing experience. GOG peddle the old outdated v1.2 as well, which makes thing worse! The only official patch still available online is v1.3, and that fixes a few issues with alien weapons being usable without their corresponding handling techs being researched. But it does not fix:-
Most of the "stat boosting items" (eg those syringe like "intellegence boosters" etc) from doing NOTHING at all. Many of the "psionic powers" and items that also do such boosts, do nothing. All lot of other things do nothing, or simply do not do what "it says on their tins".
Squad members in reserve keep on auto reloading their backpacks, which isn't convenient when they are not part of your active squad.
Province names are cross-wired, such as "Luanda / Antanarivo" and "Darwin / Port Moresby".
Not major game killers I know, but they show the sheer lack of quality assurance by Altar Interactive. This game went through NO quality checks at all during it's creation! The whole thing is just an LQ debacle! I'd say Aftershock is one the buggiest games of the naughties! Even giving Campaign and Campaign II by Empire Software (from the 90s) a run for their money - anyone whose played those will testify how bug infested they were!
Don't take drones to the battles on the alien "Myceum" ship or whatever it was called. They will go screwy squirrel in the squad UI.
There's so few maps, so repetitive, all with enemies starting in the same places. It gets boring quick. Yet the game goes on and on and on and on and on. Even when you've destroyed the "myceum" it still continues, presumably to allow you mop up the remaining Cultist controlled regions. I had no patience left with this game to do so, so if there is a final full game ending other than the "destroyed the myceum" one, I don't know.
There are no mods no longer available on-line, I notice a few in some old threads on this board. But they are all dead.
I have NO desire to even think of even looking at this game ever again!
It was too easy in some missions, and then also suffered sudden massive diffulty spikes in Cultist missions. This time, the cheats work. I used "win_mission" over and over, not because it was too hard, but I was tired of the same old maps again and again and just wanted to expedite the game along.
Oddly, first time I played it, I didn't find it so hateful, but I do remember getting pretty frustrated with it.

Now Afterlight I still have fond memories of playing a few years ago....... by far the best of the three. Aftershock is so bad, and so outdated here on GOG, they should stop charging for it, it's only worth £0.00 trust me.

Altar Interactive never were a good developer, but they excelled themselves with Aftershock!