It works like this.
If you buy The Witcher 3 you will get the 16 free DLC, no matter which version you have bought. If you buy the game before or after the game has launched, you still get the 16 free DLC.
If you have bought the retailed boxed PC dvd version, then you can download the 16 free DLC from GOG.com.
If you buy the game from Steam then you can download the 16 free DLC from Steam.
PS4 and Xbox One games, the 16 free DLC can be downloaded from PSN and Xbox Live.
The second part is the 2 paid for expansions. If you buy the PC retail boxed dvd version, then you will have to go to GOG.com to buy the expansions and download them from GOG.com.
Witcher 3 from Steam, you will have to buy the expansions from Steam and download them from Steam.
PS4 and Xbox One. You will have to buy the expansions from PSN and Xbox live and download from them.
We are still hoping that at some point in the future, the expansions and DLC will be sold at retail, meaning no downloading. But that might only happen after the expansions are released digitally.
So to answer your question. You have bought the retail PC dvd version, you install and play the game. Every week after release, you will have to go to GOG.com to download the free DLC.
The paid for expansion DLC, can be bought now from GOG.com. In 6 months the first expansion will be released and then you go to GOG.com to buy and download that expansion. The next expansion will be released next year, 2016, then you go to GOG.com to buy and download that expansion.
I hope I made it clear for you.