This is possible on Android today and has been possible since Android launched. I use Firefox as my default browser, and Gmail opens links in there. It is up to iOS to provide APIs that make this possible.

iOS may not have a central browser setting yet, but apps can still invoke other apps directly. Gmail on iOS uses that to offer a choice between Chrome and Safari, and offers to save your choice.

It just tends to regularly forget that I told it to save my choice of Safari.

On iOS, it also has no way of knowing that you have installed Firefox to give you that choice, which makes using Firefox on iOS even more of a nonstarter than it already is.

Of course not. This is why Apple didn’t allow it until now.

What are you talking about exactly?

iOS GMail App > Settings > Default Apps

You cannot choose Firefox or Brave but you certainly can choose Safari.

I believe the complaint is that I have to renew that decision regularly.

That's not true, "Safari" for iOS Gmail means a shitty in-app web view, not sharing cookies, history or UI with actual Safari, Gmail itself acting as the "browser".

This shady behavior (calling a web view "Safari") is one of the reasons I gave up on Gmail.

I can select Safari as default in Gmail?

No you can't. It's just a web view.

No.