Interesting. Google is harvesting information from Gmail about any purchases you make and then compiling them into a Purchases page.
Checking out my Purchases page, it looks like Google has a fairly accurate record of all my Amazon, Steam and (not surprising) Google Play purchases.
Honestly, I’m not sure why Google does things like this. It adds very little value to end users, Google claims they’re not using this information to target ads, and I imagine most people are going to find this creepy AF.