Checking the tier seems to be a part of the authorization layer, same as checking if the user has logged in.

I don't know how small you like your microservices, but you can either:

• create a small module within your gateway to call Stripe API and check the tier, or
• create a microservice that will act as an adapter to Stripe (or another/multiple payment processors in the future), which your gateway will call to check the tier. I would lean towards this option, if you have other reasons to call Stripe API, i.e. upgrade Subscriptions, etc.