upas/fs -f /pops/pop.gmail.com
See this message for details. That message provides a pointer to a "pop3get" program which can be used to fetch messages and subsequently deliver them to a Plan 9 mailbox.
Gmail offers an IMAP interface, however, that works nicely for upasfs(4). Something like:
upas/fs -f /email@example.com
will enable reading Gmail with Plan 9 mail tools.
In some circumstances, it may also be possible to use the Plan 9 mail tools to send via Gmail. If you change the last line of /mail/lib/remotemail to:
exec /bin/upas/smtp -sa -u firstname.lastname@example.org -h $fd tcp!smtp.gmail.com!smtp $sender $*
This will, however, send all mail on the system as that Gmail user. Also, if mail is ever queued for later delivery, the factotum visible to cron will need the keys to authenticate to Gmail.