example:

function init() {
    var aspect = window.innerWidth / window.innerHeight
    camera = new THREE.PerspectiveCamera(45, aspect, 1, 10000)
    camera.position.z = 300
    scene = new THREE.Scene()

    mesh = new THREE.Mesh(new THREE.TorusGeometry(10, 3, 16, 30),
                          new THREE.MeshPhongMaterial({ transparent: true, shading: THREE.SmoothShading }))

    mesh2 = new THREE.Mesh(new THREE.TorusGeometry(10, 3, 16, 30),
                          new THREE.MeshPhongMaterial({ transparent: true, shading: THREE.SmoothShading }))

    var material = new THREE.MeshPhongMaterial({ transparent: true, shading: THREE.SmoothShading })
    var material2 = new THREE.MeshPhongMaterial({ transparent: true, shading: THREE.SmoothShading })



    var light = new THREE.PointLight(0xffff00, 1, 0)
    light.position.set( 100, 50, 300 )

    scene.add(light)

    var loader = new THREE.OBJLoader();

    loader.load('1.obj', function(object) {
        var l = object.children.length;
        for (var i=0; i< 1; i++) {
            object.children[i].material = material
        }
        mesh = object;
        mesh.material = material;
        mesh.visible = true;
        mesh.position.set(50,0,0);
        scene.add(mesh);
    });

    loader.load('2.obj', function(object) {
        var l = object.children.length;
        for (var i=0; i< 1; i++) {
            object.children[i].material = material2
        }
        mesh2 = object;
        mesh2.material = material2;
        mesh2.visible = true;
        mesh2.position.set(-50,0,0)
        scene.add(mesh2);
    });

    renderer = new THREE.WebGLRenderer();
    renderer.setClearColor(0x007f7f, 1);
    renderer.setPixelRatio( window.devicePixelRatio );
    renderer.setSize( window.innerWidth, window.innerHeight );

    buildGUI();

    document.body.appendChild( renderer.domElement );
    window.addEventListener('resize', onWindowResize, false);
}

function onWindowResize() {
    camera.aspect = window.innerWidth / window.innerHeight;
    camera.updateProjectionMatrix();
    renderer.setSize( window.innerWidth, window.innerHeight );
}

function animate() {
    renderer.render( scene, camera );
    requestAnimationFrame(animate);
}






GUI:
function buildGUI() {
    var gui = new dat.GUI({ autoPlace: false });
    var objectsMenu = gui.addFolder('Objects:');
    var object = objectsMenu.addFolder('robot');
    var lv, lv2;

    object.add(mesh, 'visible').name('Visible').onChange((value) => {
        mesh.visible = value
    });

    var Config = function() {
        this.color = "#ffffff";
    }

    var conf = new Config();
    object.addColor(conf, 'color').name('Color').onChange((value) => {
        value = value.replace('#', '0x');
        mesh.material.color.setHex(value)
    });

    var CameraZ = function() {
        this.z = 30;
    }

    var cameraZ = new CameraZ()
    object.add(cameraZ, 'z').name('Camera z:').min(0).max(100).step(1).onChange(
        function(value) {
            mesh.position.z = value;
        }
    )

    var Transparency = function() {
        this.transp = 1;
    }

    var transparency = new Transparency();
    object.add(transparency, 'transp').name('Transparency:').min(0).max(1).step(0.01).onChange(
        function(value) {
            mesh.material.opacity = value;
        }
    )

    object = objectsMenu.addFolder('duck');

    object.add(mesh2, 'visible').name('Visible').onChange((value) => {
        mesh2.visible = value
    });

    var Config = function() {
        this.color = "#ffffff";
    }

    var conf = new Config();
    object.addColor(conf, 'color').name('Color').onChange((value) => {
        value = value.replace('#', '0x');
        mesh2.material.color.setHex(value);
    });

    var CameraZ = function() {
        this.z = 30;
    }

    var cameraZ = new CameraZ()
    object.add(cameraZ, 'z').name('Camera z:').min(0).max(100).step(1).onChange(
        function(value) {
            mesh2.position.z = value;
        }
    )

    var Transparency = function() {
        this.transp = 1;
    }

    var transparency = new Transparency();
    object.add(transparency, 'transp').name('Transparency:').min(0).max(1).step(0.01).onChange(
        function(value) {
            mesh2.material.opacity = value;
        }
    )

    var ObjectMaterial = function() {
        this.material = "";
    }

    objectsMenu.open();
    var customContainer = document.getElementById('my-gui-container');
    customContainer.appendChild(gui.domElement);
}