i have Searched few website online, they say to put in root folder, but i cant find any root folder in my project. i am using GGTS for my project and want to have a gradle build.
Root folder is not a folder named "root". It's a main folder which your project is in.
Named the same as your project. You open up that folder and put the build.groovy file right in there. the path would be .../your_project_name/build.groovy
You can actually use this tutorial to see detailed description about gradle build in GGTS.