About what exactly are your friends scared? Trust me, having an account on GOG is absolutely safe. You just choose the game you want to buy, pay immediately, for example with PayPal, and right after the transaction is done, you can download the installation files and any goodies that come with them (manuals, wallpapers etc.). So, that means GOG does not even have the money in their hands, but you can already download and play the games that you have ordered, which means there is no waiting time or anything. These files can even be burned on CD/DVD, carried on a USB stick, have (obviously) no virus or adware inside them and the best of all: You do not need any kind of internet connection to install and even play these games. Having GOG Galaxy is NOT a must (I do not have it myself for instance). So, unlike with Steam, if GOG should get killed one day, you will still have your games ready, if you have once, like I did, downloaded the installation files and burned them for your own usage on CD/DVD. There is also a refund option, but I have never needed that before, so I cannot say if it is either easy or hard to get your money back, if you are unsatisfied, for example if the game does not run properly on your system.
My only complaint may be that GOG just puts every indie game onto its platform these days, as well as even new and very expensive games, which is as if they have forgotten about their name (Good Old Games), but there are so many classics availiable here, which GOG had made running fine on modern systems, that your friends surely will find something they like, just like SWAT 4. Trust me, they will not be disappointed. I never was indeed and I am a member since February 2016, so it has been a while already.