Even if Gmail is currently delivering the messages (for now...), other service providers have to manage spam in their own way, and TLDs are sometimes a really strong signal for message reputation. For example, 99% of email traffic from .info addresses is spam, and the other 1% is mostly spam too.
Even major services. I tried to create an Esty account the other day, and it rejected all addresses from my Fastmail-managed personal domains, and would only take a gmail address.
Luckly it turns out you don't need an account to order, and I was able to use my custom domain for an unregistered checkout.
This must be related to the tld you're using, right? Not that Etsy only allows @gmail.com addresses?
I've noticed that some sites will specifically ban public domains from throwaway services like mailinator, but I can't recall having encountered a blanket tld ban.
I wasn't using a weird TLD. It was a .us that only my family uses.
I didn't try too hard to get it to work, or figure out exactly what their criteria were, but I kept getting blocked until I used my old gmail. Their customer support wasn't very helpful, either.