Hello and welcome to Android Forums. Hopefully I can help you with your problem.
Without fully understanding your question and what you've done with your phones. The best option I can offer is to sign into your Gmail account via a computer and change your password.
Then....
Reboot your phone and you will be prompted by your device to enter the correct password for your Google Play account. After entering the correct password, you should then be able to log into the Google Play app. Do so and make sure you have Google Play access. Then navigate to your Gmail app and try opening it. You may be prompted to sync the app.... you can do so by selecting the settings tab in the app options. Do it. If it still will not sign you in then back out of Gmail and tap the Google Play app again. Do a search for Gmail. Choose that option that reads UNINSTALL and Gmail will be uninstalled from your S2.
Reboot your device and let it boot up completely. Tap the Google Play app again and search for Gmail. You will be given the option to install it. Do so. Let the Gmail app install completely. After it has installed, open Gmail. It will probably ask you to sync the app. Tap your settings in the app and select refresh or sync, which ever option is available. Give it a minute to sync with your Google account and you should be good to go.
There are other easier ways to try and solve your problem but as I mentioned above, with the info provided, I do not fully understand what you've done to cause the issue and why your Gmail app isn't syncing with your phone. In all actuality, once you are signed into Google Play, your Gmail app should work after it has synced. If by chance something has went wrong and the Gmail apps have become corrupt, the instructions I have provided will solve that issue.
Good luck and I hope you get it fixed.
Another option you have is to join Android Forums. We have a wealth of information pertaining to the S2 device. I invite you to join, meet the awesome members of that section, and learn even more about your devices full potential.