If you don't find the auto spelling correction is effective then you can perform several actions to solve the problem. Here are some settings that allow you to make it more effective.
1. Touch the icon Settings.
2. Touch the Language & input.
3. Tap the Settings next to Google Keyboard.
4. In the zone Text Correction , uncheck Block offensive words not to the spell checker words it considers "coarse".
5. Touch the option Auto Correct and select Proactive so that the correction is more "aggressive".
That's it guys!