Here's some advice I recently gave to someone for steps to take after getting hacked. If you have any questions, feel free to email me with them at my gmail accout of chinatablet.
Assume they used your gmail account to get OTHER passwords, such as facebook, twitter, bank, amazon, etc. (All they have to do is click "lost my password" on another site and it send an email to your gmail account -- assuming you used the gmail account to register there). I actually have a different email account for some sensitive registrations.
Change ALL of those passwords too. Especially Apple ITunes, Amazon, other online catalogs, etc.
Use a password manager that stores on your local computer, such as Roboform (paid), or LastPass (free). Never use same password on different services.
If you find any evidence of other activity, consider alerting Google to it as well. They might have further logs to chase down hackers.
Gmail has a new "two factor" authentication system that you should take advantage of. "Authentication" basically means proving who you are, by typing in a password, etc. "Two factor" means that you have to prove who you are in two steps whenever you sign in on a new computer (you can still tell it to remember you). One example of a second method of proof would be if Gmail sent you an SMS code on your phone. A hacker wouldn't normally have your phone as well. Of course, that does mean that you shouldn't store passwords on your phone, in case your phone get's stolen!
In fact, please get in the habit of changing your email every 45 days or so. According to the FBI, stolen passwords often aren't used right away. Changing your passwords every 45 days or so on your important accounts could put a hacker back on the outside before he does anything nasty.
Consider signing up for Google's free "Account Activity Report", which is available once a month.
You can also read Googles own "Gmail Security Checklist" for more ideas.
There are TWO locations. Click your name for access to your general Google account. Click the GEAR icon for the specific gmail settings.
Click the "Details" link to get to the Recent Activity page. Scroll back through previous days if available.
Be sure to sign out other sessions. There's also (not shown) a list of any other currently active sessions. This would include your iPhone, tablet, etc. so don't assume the worse if you have multiple sessions, but sign them all out here, just to be sure!
Click the link to activate the activity alert. Note that it takes a week to disable this, so that hackers can't immediately turn it off on you!