Hello,
I don't know if there's still much people using Mac OS 9, but as far as
I'm concerned I can't afford to upgrade to OS X. For what I know, the
only browser supporting Gmail that works on System 9 is WaMCom 1.3.1, a
variation of Mozilla 1.3.1, with some extensions of Mozilla 1.4. You
can download it at :
http://wamcom.org/latest-131/