SPA is just an abbreviation of "single page application" and only means a web app that doesn't do full page reloads, it doesn't require the use of any specific framework, so Gmail definitely qualifies and it used it from version one. It wasn't even the first, XMLHttpRequest was created by Microsoft for Outlook Web Access and it shipped in 1999 in IE5. Before Gmail there was also Oddpost, another SPA webmail app, so people started using it to build non-reloading web apps almost immediately. Gmail was the iPhone of web mail: not the first, no real new tech, but just very well done overall and popularized the concept.

You seem to be trying to redefine SPA to mean something else and much vaguer - the use of some specific frameworks, or not being "hacky" whatever that means - but your history here is just off. Reality is the web has changed relatively little since 2008. It got some more APIs that weren't new, they were used in native desktop/mobile apps for many years beforehand, and that's about it.