X-Com and XCOM are different series, as XCOM is a more recent reboot.

GOG now has the X-Com series, the classic games of old. As CARRiON.FLOWERS says, the first three games are long and slow and can be hard and unforgiving. I did once finish the first game, but the second game beat me after I'd progressed quite some way, whilst the third game I've not yet given a proper chance to play through. Interceptor mixes up strategy (base building, research, etc) with space combat. For me it got boring quickly so I cheated in order to just play through the story missions. Then lastly there is Enforcer which I don't even think is considered canon, where you play a robot (on your own) killing the invading aliens, all done in a third person shooter.

The XCOM reboot series is on Steam (we don't know if it will come to GOG, we'll have to wait and see) and consists of XCOM: Enemy Unknown (+ Enemy Within DLC), The Bureau: XCOM Declassified and XCOM 2.

The Bureau is a prequel of sorts, and is a strategic third person shooter game where you manage a squad consisting of yourself and two others. I recently played through this game.

XCOM: Enemy Unknown was the first of the XCOM series released and is a reboot of the original X-Com: UFO Defense (which was also known as UFO: Enemy Unknown in Europe). The game is still a squad based strategic game, but is simplified in a number of ways. I've only played the first few missions, but from what I've seen, certainly on the easier difficulties, it is more forgiving.

As for XCOM 2 (which is a sequel to XCOM: Enemy Unknown), I've not played it, but I've heard that it is very hard. Same type of game, though.

So, for short bursts you might want to look at the reboot series, but those aren't here on GOG at the current time. That said, if you use OpenXcom with UFO Defense (and also Terror From The Deep if you use the nightly build of OpenXcom) then you can actually play one off battles in the strategic turn based play, rather than doing the full campaign and research and base management and all that. This is really quite nice when you just want to have a quick one-off battle with the aliens.

And - as promised - bought! I do own every game from this bundle at least once elsewhere but the deal is too good to miss. I have to celebrate!

Get Xcomutil, it has autoresolution for combat at any point you want. Can skip the entire battle, or kill a few then skip (for better odds), or just skip searching for the last few aliens.

Does someone know if the following two games (Interceptor and Enforcer) supports Full HD (1080p) resolution? Thanks in advance!

The main reason to buy Enforcer and Interceptor is... that there is no reason.

Well I want only to play a popcorn action game nad Enforcer seems to be the best candidate for that.

The running off bit was most likely an alien psionic attack, though it can happen to a soldier with very low bravery when you lose a soldier or two. It's a good idea to hire a large number of soldiers at once, and then screen them one by one and sack those with very bad stats, especially those with low reactions and very low bravery.
Psionic attacks are trickier, because it hinges on a hidden stat that you only discover fairly far into the game. There are some strategies to deal with the issue though http://www.ufopaedia.org/index.php/Psionic_Strategies

All in all a great game, but there's no escaping the fact that it's a very time-consuming game. Land based missions need to be approached with the utmost care, unless one resorts to save-scumming, but that just makes it boring and pointless.

If you do want to check it out again, look at UFOextender or openxcom, since the original is far to archaic at this point imo

For me it's true for music..It's just becomes the game so well. ..And it's also one of those rare occasions (althought some may disagree) where FM music beats GM/MT32/GUS-one. All those electronic, eerie sounds so similar to the ones you could hear in classic sf movies..ahh...

While I haven't played the classics(only the new XCom EU(2012) with EW and all DLCs) I'm happy that they're finally here. I do hope that the new ones will be available DRM free as well some day.

As for the comparisons to the old ones, the old ones are definitely harder, EU does have plenty of difficutly settings, and due to how some game mechanics work, you do get increased difficulty. According to people, Xcom2 got easier, however I haven't played that.

I do know some things about the classics and there are definitely mechanics which I liked more from them, for example in the new games you basically have two moves, so you can either run twice or run and then fire(firing automatically ends the turn unless you have certain abilities) quite limiting what you can do. In the original you had time units, so if I understood it right, each action would cost a certain amount of that.

Then there's the hit percentage, you get a % chance to hit, so in the end you either miss or hit which is a bit odd given how some weapons fire like 7 to 8 times. A Class in the game has a lower hit chance to offset the damage I guess, which made them way too unrelieable to use for me, and then there's also things like enemy defense, which you would thought reduce critical strike chance but instead reduce aim.

Don't get me wrong, I do love XCom EU/EW, and can see why people miss certain things from the classics(a mixed game would probably be a lot of fun).

Thanks for these, pal.
I'm new into these games (I mean, post the original X-COM) and didn't even remember there were 3rd person shooter games in the series. Great ones, actually.