I lost my android mobile, but I haven't logged out of gmail account, since using the device, I must log on. So if someone take it, he/she may access my gmail account. What should I do now?
Log-in to your gmail account. Change your password first. Then, on the page where you see mails, down in the bottom, Click on the link "Details" in the right bottom corner. Click on "Sign-out all other sessions". You will sign-out of all the sessions. It will be effective on logged-in mobile once user tries to do any action on your gmail.
Hope it helps...
As suggested by Narayanan in his above comment, I would go with Android Lost. You can directly push this app to your phone via Google Playstore's website (logging into it with your Google Account). Doing so, you can then have your device report its location -- maybe you just have misplaced it and can easily pick it up. Knowing its "round about location" and being near it, you can even have it make noises to better locate it. Finding it "wandering around" in an unusual area (suggesting someone picked it for himself), you even can issue a "remote wipe", so everything will be cleaned from the phone.
Watch the video on the app's Playstore page to see more!
As a second step it could be useful to change your passwords etc., as described by shubansh's answer. However, disconnecting your device from your account should not be done before you evaluated my above suggestion -- because doing that first will rob you of the chance to use things like Android Lost or Plan B.