You need to change this :
"version = "1.0" group = "com.yourname.modid" // http://maven.apache.org/guides/mini/guide-naming-conventions.html archivesBaseName = "modid""
Change "group" to the package of your mod, for example "fr.leonard.anmod" And change "modid" to your mod id. For example "ic2". You can change "version" everytime you export your mod to a new version.
When you have modified the build.gradle, do grade build in the terminal. You can find your mod in an folder (I don't remember).