I purchased Horizon 3 Ultimate digital code at Gamestop but it doesn't include 2015 ...

You would still need to speak to Gamestop about it. It's a retailer-specific promotion.

I bought the digital version and I bought it online. There was no other code except for the code to the Ultimate version of game.

You would need to speak to Gamestop about this. They are the ones who should have provided you with the code.

It's possible the code is printed on your receipt rather than placed inside the game case, but if not you will want to speak to them directly.

i can't find a listing for the Windows version, but the Xbox One listing says it's going to use Play Anywhere, so one purchase should work on both systems.

www.microsoft.com/.../bpsnd9ttldmw

"Forza Horizon 3 Ultimate Edition includes the full game of Forza Horizon 3, Early Access, the Forza Horizon 3 Car Pass, VIP Membership and cars, Forza Hub App, and the Motorsport All-Stars Car Pack, which gives you the chance to take 10 of the hottest modern
race cars off the track and out on the open road!"

"ULTIMATE EDITION OWNERS WILL HAVE EARLY ACCESS TO THE GAME ON 9/23/16. PURCHASE THIS OFFER BEFORE 9/27/16 AND RECEIVE 2 CAR PACKS AT NO ADDITIONAL CHARGE: Forza Horizon 2 Rockstar Car Pack (Xbox One) and Forza Motorsport 6 Meguiar's Car Pack (Xbox One)."

"Forza Horizon 3 VIP members receive five exclusive VIP cars, the 2016 Lamborghini Aventador LP750-4 SV, 2016 Koenigsegg Regera, 2015 Ferrari F12tdf, 2015 Ultima Evolution Coupe 1020, and 2015 Ford Falcon GT F 351. VIPs will receive additional benefits,
including double Wheelspin earnings, access to exclusive online events, community gifts, and more."

"The Motorsport All-Stars Car Pack for Forza Horizon 3 is your chance to take 10 of the hottest modern race cars off the track and out on the open road" (it doesn't specify on that page which cars are included.)

"The Car Pass brings you six monthly car packs (7 cars each) beginning in October 2016, ending March 2017. That’s 42 amazing new cars total."

hopefully that answers your question. let us know if you need more assistance.

Depends on the content of the Ultimate Digital Edition. If the game came with promo codes that you had to redeem then you should be in the clear with just purchasing the standard digital copy of Forza H3. From what I can tell the car packs are standalone
to the game which anyone can purchase when and if it is released by the publisher. So I think it may be safe to assume that you will be able to keep your extras from the ultimate edition when you move to the digital version.

You can check out the forza horizon 3 forums here to see if you can get a definitive answer

forums.forzamotorsport.net/turn10_topics91_Forza-Horizon-3-Support----Xbox-One.aspx

Cheers!