First stab

app/build.gradle.kts

@@ -0,0 +1,45 @@
+import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
+
+plugins {
+    kotlin("jvm") version "1.3.21"
+    application
+}
+
+group = "org.nixos"
+version = "1.0.0-SNAPSHOT"
+
+application {
+    mainClassName = "org.nixos.gradle2nix.MainKt"
+    applicationName = "gradle2nix"
+    applicationDefaultJvmArgs += "-Dorg.nixos.gradle2nix.initScript=@APP_HOME@/gradle/init.gradle"
+    applicationDistribution
+        .from(tasks.getByPath(":plugin:shadowJar"))
+        .into("gradle")
+        .rename("plugin.*\\.jar", "plugin.jar")
+}
+
+dependencies {
+    implementation(kotlin("stdlib-jdk8"))
+    implementation("org.gradle:gradle-tooling-api:${gradle.gradleVersion}")
+    implementation("com.github.ajalt:clikt:1.7.0")
+}
+
+repositories {
+    jcenter()
+    maven { url = uri("https://repo.gradle.org/gradle/libs-releases") }
+}
+
+tasks {
+    val startScripts by existing(CreateStartScripts::class)
+    startScripts {
+        doLast {
+            unixScript.writeText(unixScript.readText().replace("@APP_HOME@", "\$APP_HOME"))
+            windowsScript.writeText(windowsScript.readText().replace("@APP_HOME@", "%APP_HOME%"))
+        }
+    }
+    withType<KotlinCompile> {
+        kotlinOptions {
+            jvmTarget = "1.8"
+        }
+    }
+}

app/src/dist/gradle/init.gradle

@@ -0,0 +1,7 @@
+initscript {
+    dependencies {
+        classpath files("plugin.jar")
+    }
+}
+
+apply plugin: org.nixos.gradle2nix.Gradle2NixPlugin

app/src/main/kotlin/org/nixos/gradle2nix/GradleRunner.kt

@@ -0,0 +1,45 @@
+package org.nixos.gradle2nix
+
+import org.gradle.tooling.GradleConnector
+import java.io.File
+
+class GradleRunner(
+    private val projectDir: File,
+    private val useWrapper: Boolean,
+    private val gradleVersion: String?,
+    private val configurations: List<String>
+) {
+    companion object {
+        val initScript: String = System.getProperty("org.nixos.gradle2nix.initScript")
+    }
+
+    fun runGradle() {
+        GradleConnector.newConnector()
+            .apply {
+                if (useWrapper) {
+                    useBuildDistribution()
+                } else if (gradleVersion != null) {
+                    useGradleVersion(gradleVersion)
+                }
+            }
+            .forProjectDirectory(projectDir)
+            .connect()
+            .use { connection ->
+                connection.newBuild()
+                    .withArguments("--init-script", initScript)
+                    .apply {
+                        if (configurations.isNotEmpty()) {
+                            withArguments(
+                                "-Dorg.nixos.gradle2nix.configurations=${configurations.joinToString(
+                                    ","
+                                )}"
+                            )
+                        }
+                    }
+                    .forTasks("nixGradleEnv")
+                    .setStandardOutput(System.out)
+                    .setStandardError(System.err)
+                    .run()
+            }
+    }
+}

app/src/main/kotlin/org/nixos/gradle2nix/Main.kt

@@ -0,0 +1,25 @@
+package org.nixos.gradle2nix
+
+import com.github.ajalt.clikt.core.CliktCommand
+import com.github.ajalt.clikt.parameters.arguments.argument
+import com.github.ajalt.clikt.parameters.arguments.defaultLazy
+import com.github.ajalt.clikt.parameters.options.flag
+import com.github.ajalt.clikt.parameters.options.multiple
+import com.github.ajalt.clikt.parameters.options.option
+import com.github.ajalt.clikt.parameters.types.file
+import java.io.File
+
+class Main : CliktCommand() {
+    val wrapper: Boolean by option(help = "Use the project's gradle wrapper for building").flag()
+    val gradleVersion: String? by option(help = "Use a specific Gradle version")
+    val configurations: List<String> by option(help = "Project configuration(s)").multiple()
+    val projectDir: File by argument(help = "Path to the project root")
+        .file(exists = true, fileOkay = false, folderOkay = true, readable = true)
+        .defaultLazy { File(".") }
+
+    override fun run() {
+        GradleRunner(projectDir, wrapper, gradleVersion, configurations).runGradle()
+    }
+}
+
+fun main(args: Array<String>) = Main().main(args)