Direct purchase: Impossible. Gamerpoints (the score from achievements) are a reputation metric, not a currency. You cannot spend them in the Microsoft Store.
Microsoft Rewards (formerly Bing Rewards) lets you redeem points for Xbox Gift Cards. Here's the loop:
Clarification: Gamerpoints ≠ Microsoft Rewards points. They're separate systems. You earn Rewards points by using Bing, shopping at the Microsoft Store, completing Game Pass quests, and weekly sets. Gamerpoints just track achievement progress.
Bottom line: Achievements build bragging rights; Rewards points build wallet credit. Don't confuse the two.