Dennis Hwang designed the Gmail logo. At the time, Dennis designed virtually all of the Google doodles and he did a lot of the new logo work as well.
The logo was designed literally the night before the product launched. We were up very late and Sergey and I went down to his cube to watch him make it.
The initial version used the same font as the Google logo (Catull), but Catull has a very awkward 'a', so Dennis decided to use Catull for the 'G' to tie the brand to Google, then cast the others in a cleaner sans-serif (Myriad Pro, if I recall correctly).
Another ex-Googler, Douglas Edwards, confirms the story in his book "I'm Feeling Lucky: The Confessions of Google Employee Number 59":
Dennis Hwang spent the day before the launch coming up with ideas for a logo and trying to make it work in conjunction with the clown-colored Google brand. (...) Even after four years at Google, I found it astounding that one twenty-something guy was sitting alone at his desk, sipping tea and developing the main branding element for a product to be used by millions of people - the night before it was scheduled to launch.