Originally Posted by: NorCalNavyMike 
Typically a few times per year, Games with Gold will offer a "Complete Add-Ons" collection for FH2. The pack is included in that…
This ^ is wrong. The Complete add-ons collection includes 17 items with price from the store and 1 free item from the store.
It doesn't include the 10th anniversary pack (not sold in the store), the Pre-sale pack (sold in the store) and Day One pack (not sold in the store) .
And pointlessly includes 1 of the 13 free packs available in the store, but the free packs are not needed for those with the Car Pass (which itself is in the add-ons pack) (except for Mazda and Launch bonus pack, both should be downloaded even with the Car pass).
The ways to get the content not included in the store is redeeming a code included in the box (Day One and 10th anniversary), or
10th anniversary is included in any of the digital versions of the game in the store, be it one of the three standalone versions or the bundled versions with either Forza 5, Forza 6 or FH3.
(Day one used to be in a digital version labeled as Day One or Ultimate Day One, but just in 2014 if I'm not mistaken)
I bought a digital version on a sale just for this pack. Should have tracked a disc version with the 10th logo, but It was cheap and now I don't need to use the disc, so it was not totally a waste.
People willing to get everything still need to buy the Pre-sale pack and manually download the free Mazda and Launch bonus packs.
And redeem a code for Day One pack. (only way to get this today)
And redeem a code for 10th year pack or buy any digital version for it.