I have just upgraded (i.e., installed a full new version) my Android Studio from 1.5 to 2.0. The upgrade seemed to go smoothly, but I am having trouble with my project when
I had the same problem as yours. Fortunately, I managed to solve it. You need to go to File >> Settings >> Build, Execution, Deployment >> Build Tools >> Gradle. You need to change the Gradle Home. Mine was "C:/Program Files/Android/Android Studio/gradle/gradle-2.08" and it said the path is invalid. So, just recheck your path in your file explorer and change it. I changed mine to "C:/Program Files/Android/Android Studio/gradle/gradle-2.10".