project_root
 |-- app/
 |   |-- src/
 |       |-- main/
 |           |-- java/
 |               |-- com.company.stuff/
 |                   |-- controller
 |                       |-- fileThatUsesGson.java
 |-- client/
     |-- libs/
         |-- gson-2.8.1.jar

dependencies {
    implementation fileTree(include: ['*.jar'], dir: 'libs')
    implementation project(path: ':client')
}

dependencies {
    implementation fileTree(include: ['*.jar'], dir: 'libs')
    implementation files('libs/gson-2.8.1.jar')
}