With tens of millions of users, Gmail is arguably one of Google's most successful projects to date. Besides just being cool and usually fast to access, another of the reasons Gmail has done so well is because of how it retains cross-browser and cross-platform functionality with people being able to access most of Gmail features regardless of what OS or browser they are using.
Considering the challenges to develop for legacy browsers that barely keep up with standards, it seems that Google has "preferred" browsers in mind and is officially steering people away from using Internet Explorer with Gmail.
Focusing on Internet Explorer 6 only, Gmail now serves up messages to IE6 users when they log in, telling them that by switching to a browser like Chrome or Firefox 3, their speed when interacting with Gmail will be "twice as fast". The company pulled the same message from being displayed to IE7 users. Yet the fact they are serving up those messages at all is relevant as IE6 still represents a substantial portion of the browser market today - about 14% of users visiting TechSpot still use IE6, which is surprising when you take into account this is a tech-centric publication.
In this particular instance, steering people away from the aging browser doesn't seem to be a bad thing at all. Even Microsoft attempts to upgrade people to IE7 at every turn. Consider, however, that Google might decide to take this further in the future by offering select features to a handful of browsers of its liking, which wouldn't be as a welcomed move from our perspective.