You should add the path to junit-platform-gradle-plugin to a classpath, and add test dependencies to the appropriate section in the build.gradle file. As a result of this, your build.gradle file will look as follows:

buildscript {
    dependencies {
        classpath 'org.junit.platform:junit-platform-gradle-plugin:1.0.0'
    }
}

apply plugin: 'org.junit.platform.gradle.plugin'

junitPlatform {
    filters {
        engines {
            include 'spek'
        }
    }
}

repositories {
    maven { url "http://dl.bintray.com/jetbrains/spek" }
}

dependencies {
    testCompile 'org.jetbrains.spek:spek-api:1.1.5'
    testRuntime 'org.jetbrains.spek:spek-junit-platform-engine:1.1.5'
}