I remember Internet Explorer also doing this back in the day. When I was a kid I assumed it was showing correct progress, and I'd be patiently waiting for no payoff. Only much later I learned it meant nothing, it was just showing it wasn't dead.

I'm not sure about GMail's case, but I recall Mac OS X (back when it was called that) used to do something like this. Since showing a correct progress bar doesn't make it look so nice (it might be stuck at 90% for longer than it took to get there) and might be difficult to estimate, they instead wrote the last boot time into a file and on the next boot would render a smooth progress bar that would follow the timer. In my experience it made for a reasonable estimation of the boot time and I thought it was a clever solution.