Steam requires the client. GOG provides a 245MB offline installer. You can burn this to a CD, put it on a USB stick, or stash it in a cloud drive. In 10 years, when Steam has changed its terms or if you are on a submarine with no internet, the GOG version will still run. This aligns perfectly with the game’s theme of isolation in space.
The "Advanced Edition" is not DLC; it was a free update. But for those who bought the GOG version, it is seamlessly integrated. Here is what the Advanced Edition injects into the base game: FTL - Faster Than Light Advanced Edition 1.5.13 GOG
In 1.5.13, enemies target your systems intelligently. Never upgrade your Oxygen past level 1 unless you have spare scrap. Instead, put that scrap into Engines. A 45% dodge chance is better than a third shield bubble against missiles. Steam requires the client