It wasn't gmail -- it was google maps that really made everyone look again at javascript. Although MS added XMLHTTPRequest, it took something like google maps that pushed it further than anyone thought at the time to make JS a real thing.
Yeah, gmail was different than other webmail products but its innovation was really on the backend, giving everyone 1000x more quota than Yahoo was giving them. On the other hand the google maps frontend was what really sold it. Up to then you were getting a full page load every time you panned or zoomed mapquest or yahoo maps. Gmail did not eradicate yahoo mail or hotmail. Google maps promptly eradicated mapquest.
