This should happen automatically if:
-- your Gmail account and your Droid account have the same @gmail address, and
-- you have your Droid set to Auto-sync (from Droid home page, click the Menu button, then choose Settings, then choose "Accounts & sync," and check the "Auto-sync" box (green check-mark should appear).
By the way, this is something you can actually get from the Droid user manual, available as a PDF manual via the Motorola website (and I think from the Verizon Wireless website as well).