Adding an additional test suite to Gradle

the "integration" sourceSet has not configured its compile and runtime classpath. That's why it can't find the classes from your main sourceset. you can configure the compile and runtime classpath in the following way:

sourceSets {
    integTest {
        java.srcDir file('src/integration-test/java')
        resources.srcDir file('src/integration-test/resources')
        compileClasspath = sourceSets.main.output + configurations.integTest
        runtimeClasspath = output + compileClasspath
    }
}

In most cases you want to use the same dependencies as your unit tests as well as some new ones. This will add the dependencies of your unit tests on top of the existing ones for integration tests (if any).

sourceSets {
    integrationTest {
        compileClasspath += sourceSets.test.compileClasspath
        runtimeClasspath += sourceSets.test.runtimeClasspath
    }
}