The Season Pass has a 30% discount on Steam until September 22.
Now, let me explain something.
Let's say that you took advantage of one of the GMG coupons for the base game and purchased it from there. Naturally, this means that you don't own the game on Steam yet. If you attempt to buy the Season Pass from Steam, you will get an error message saying that you need to own the base game first before you can purchase the DLC. So what to do?
Here is how you get around that: purchase the 30% discounted Season Pass as a gift and store it in your Steam Inventory. Then when you activate the base game, all you have to do is add the Season Pass to your Steam Library.