Yes, using an Xbox One key for Xbox Live Anywhere can let you download games on Windows 10.
Yes, using an Xbox One key for Xbox Live Anywhere can let you download games on Windows 10.
Checking the listings shows prices for those keys on DLCompare. Whether you can get them on Windows 10 depends on the retailer's support for that platform.
It seems you're confident about playing Forza Horizon 3 on both systems using one button. You're curious about Gears of War but unsure if it would change that.