Would you believe if I said that GMail does not provide an email address to report spam? After some investigation I found how you can report GMail spam to Google.

You have received spam from GMail user in your GMail account

This one is simple to address. Check the message in your GMail Inbox and click on Report Spam.

You have received spam from GMail user in your non-GMail account

First check that the spam is really from Google by checking your email headers. Chances are that the email is from a different spam source (example) faking as GMail. However if it is really from GMail then report it here.

Note: While it may look like the form above should used to submit phishing emails only, the header says - "I would like to report a Gmail user who has sent messages that violate the Gmail Program Policies and/or Terms of Use.", which is appropriate for reporting spam.

You should also forward the spam email as attachment to spam@uce.gov, the US government address to report spam. Be polite, they haven't spammed you; they are simply trying to help you.

Frankly I think GMail should have a direct email address for reporting spam. Not having a direct abuse email address makes people think that they are spammer friendly.