Centering text in CSS grid

Question

Could you help me in aligning the number 6 vertically in the div, using only CSS-grid?

I'm learning CSS-grid and my idea is to be able to center and align text horizontally as well as vertically. It often gives me problems. This time I need to do it entirely on CSS-grid. Have you got any ideas?

No absolute or relative positioning... only CSS-grid.

Here the code so you can give it a try:

.grid-container {
  display: grid;
  grid-template-columns: auto auto auto auto auto auto auto;
  grid-gap: 10px;
  background-color: #2196F3;
  padding: 10px;
}

.grid-container > div {
  background-color: rgba(255, 255, 255, 0.8);
  text-align: center;
  padding: 20px 0;
  font-size: 50px;
}

.item6 {
  grid-area: 1 / 6 / 3 / 8;
}

<div class="grid-container">
  <div class="item1">1</div>
  <div class="item2">2</div>
  <div class="item3">3</div>
  <div class="item4">4</div>
  <div class="item5">5</div>
  <div class="item6">6</div>
  <div class="item7">7</div>
  <div class="item8">8</div>
  <div class="item9">9</div>
  <div class="item10">10</div>
  <div class="item11">11</div>
  <div class="item12">12</div>
  <div class="item13">13</div>
  <div class="item14">14</div>
  <div class="item15">15</div>
  <div class="item16">16</div>
</div>