The 2018 Porsche 911 GT2 RS can be bought by normal means, but will not be available right away.
The car is in the Collector Tier 4 Bracket, so you will have to level up your Collector Tier before you are allowed to purchase it.
When you reach Collector Tier 4, the car can be bought for 315,000 CR. Buy the car and the Achievement will unlock immediately.
It may also be available from level-up rewards and Prize Crates, but neither is guaranteed. Once the Auction House becomes available you can also buy it from there although the price will vary.