It's not a standalone. Well... It is, and you can get it free and own it, but yes you need FO4.
The link in GOG and the launcher does not work, and it is a pain to get running. So here is my quick guide.
1. Download FO4. Either Steam or GOG.
2. Go and get FOLON from GOG.
3. You need the game downgrader, as when Bethesda updated it around the TV show time it broke FOLON. So you need an account on Fallout Nexus.
4. Install the downgrader. You need to log into your Steam account through it. You also need to 2fa through it too like you do on Steam when signing in on a new device.
5. Run the downgrader. This takes ages, as it has to contact the Nexus servers and download all of the old meshes and textures FOLON needs.
6. Make sure you have Steam running.
7. Open GOG and launch FOLON.
It should then run. However, there are issues. Most notably as spotted by me and my pals.
There are obvious audio issues. I had no sound to intermittent sound at the menu trying to watch the intro.
It will CTD.
There are texture and mesh issues on certain buildings I have seen like the bank. (no spoiler, the bank is an empty building).
Also. It is very, VERY hard. Even on Very Easy the enemies are extremely clever and you will die a lot. As an example to this. The loot in boxes and filing cabinets is total crap. In four hours I found two Stimpaks, and one of those was in the room where I spawned at the start. So I went into this erm, "building" (I won't post spoilers don't worry) and died 8 times by headshot from enemies. So I decided to get on the roof and again no spoilers, the "raiders" (forget what they call them in game, hooligans?) hide and wait until your head comes up on the ladder and headshot and kill you, and you can't stop on ladders or go back down.
So yeah, it is like playing a whole new game, but it is waaaayy more like FO3 than any other Fallout game I have played since.
Worth downloading and playing tbh, even if you don't really like FO4. It's just so extremely different and alien feeling it may be your game. Like really, Bethesda should have left the states years ago and done something new like this. Even though it clearly runs in the FO4 engine it is worlds apart.
So yeah, hopefully they will fix the bugs but even if they don't I am recommending it. I put in four hours yesterday and they were by far my best four hours in Fallout universe since FO3.
Also, look out for the small details like the animation when you get out and put away your Balisong. It's just epic.