Undefined Index: Gambar in proses_tambah on line 11

I have a problem in PHP and MYSQL codes. When i creates CRUD system. Create Read are run at usual. But, upload image didn't work. It's show error at this picture below: error images

This is the processing PHP code:

    <?php
include 'koneksi.php';

$nama_barang = $_POST['namabrg'];
$jumlah_barang = $_POST['jumlah'];
$brg_masuk = $_POST['brgmsk'];
$brg_bagus = $_POST['bagus'];
$brg_perbaikan = $_POST['perbaikan'];
$brg_rusak = $_POST['rusak'];
$brg_ket = $_POST['ket'];
$gambar = $_FILES['gambar']['name'];

if($gambar != "") {
    $ekstensi_diperbolehkan = array('png','jpg');
    $x = explode('.',$gambar);
    $ekstensi = strtolower(end($x));
    $file_tmp = $_FILES['gambar']['tmp_name'];
    $angka_acak     = rand(1,999);
    $nama_gambar_baru = $angka_acak.'-'.$gambar;
    if(in_array($ekstensi, $ekstensi_diperbolehkan) === true) {
            move_uploaded_file($file_tmp, 'tambah/gambar/'.$nama_gambar_baru);
            $query = "INSERT INTO barang (nama,jumlah,brg_msk,bagus,perbaikan,rusak,keterangan,gambar) values ('$nama_barang','$jumlah_barang',
            '$brg_masuk','$brg_bagus','$brg_perbaikan','$brg_rusak','$brg_ket','$nama_gambar_baru')";
            $result = mysqli_query($koneksi, $query);
            if(!$result){
                die ("Query gagal dijalankan: ".mysqli_errno($koneksi).
                    " - ".mysqli_error($koneksi));
            } else {
                echo "<script>alert('Data berhasil ditambah.');window.location='tambah.php';</script>";
            }
        } else {
            echo "<script>alert('Ekstensi gambar yang diperbolehkan hanya JPG dan PNG');window.location='tambah.php';</script>";
        }
    } else {
        $query =  "INSERT INTO barang (nama,jumlah,brg_msk,bagus,perbaikan,rusak,keterangan,gambar) values ('$nama_barang','$jumlah_barang',
        '$brg_masuk','$brg_bagus','$brg_perbaikan','$brg_rusak','$brg_ket',null)";
        $result = mysqli_query($koneksi, $query);
        if(!$result) {
            die ("Query gagal dijalankan: ".mysqli_errno($koneksi).
            " - ".mysqli_error($koneksi));
        } else {
            echo "<script>alert('Data berhasil ditambah.');window.location='tambah.php';</script>";
        }
    }

Here is the form code:

<form class="contact100-form validate-form" method="POST" action="proses_tambah.php">
                <div class="wrap-input100 validate-input" data-validate="Name is required">
                    <span class="label-input100">Nama Barang:</span>
                    <input class="input100" type="text" name="namabrg" placeholder="Masukkan nama barang" />
                    <span class="focus-input100"></span>
                </div>

                <div class="wrap-input100 validate-input" data-validate = "Valid email is required: ex@abc.xyz">
                    <span class="label-input100">Jumlah:</span>
                    <input class="input100" type="number" name="jumlah" />
                    <span class="focus-input100"></span>
                </div>

                <div class="wrap-input100 validate-input" data-validate="Phone is required">
                    <span class="label-input100">Barang masuk:</span>
                    <input class="input100" type="date" name="brgmsk" />
                    <span class="focus-input100"></span>
                </div>

                <div class="wrap-input100 validate-input" data-validate = "Message is required">
                    <span class="label-input100">Bagus:</span>
                    <input class="input100" name="bagus" type="number" />
                    <span class="focus-input100"></span>
                </div>

                <div class="wrap-input100 validate-input" data-validate = "Message is required">
                    <span class="label-input100">Perbaikan:</span>
                    <input class="input100" name="perbaikan" type="number" />
                    <span class="focus-input100"></span>
                </div>

                <div class="wrap-input100 validate-input" data-validate = "Message is required">
                    <span class="label-input100">Rusak:</span>
                    <input class="input100" name="rusak" type="number" />
                    <span class="focus-input100"></span>
                </div>
                <div class="wrap-input100 validate-input" data-validate = "Message is required">
                    <span class="label-input100">Keterangan:</span>
                    <input class="input100" name="ket" type="text" placeholder="Masukkan keterangan!" />
                    <span class="focus-input100"></span>
                </div>

                <div class="wrap-input100 validate-input" data-validate = "Message is required">
                    <span class="label-input100">Gambar Produk:</span>
                    <input class="input100" name="gambar" type="file" required />
                    <span class="focus-input100"></span>
                </div>

                <div class="container-contact100-form-btn">
                    <button class="contact100-form-btn">
                        <span>
                            Submit
                            <i class="fa fa-long-arrow-right m-l-7" aria-hidden="true"></i>
                        *</span>*
                    </button>
                    <a href="../invbrg.php" style="margin-left:15px;" class="contact100-form-btn">
                        <span>Back</span>
    </a>
                </div>
            </form>

MySql database are set below: SQL Database Picture

Here is the photo of my code: Processing PHP Code Form Code

All your help appreciated. Thank you.