How do I create a flexible grid layout without using CSS frameworks?

<!-- BEGIN COPY / PASTE -->
    <style>
      .grid-container {
        display: grid;
        grid-template-columns: repeat(auto-fill, minmax(100px, 1fr));
        gap: 10px;
      }
      .grid-item {
        background-color: #f2f2f2;
        padding: 20px;
        text-align: center;
      }
    </style>

    <div class="grid-container">
      <div class="grid-item">Item 1</div>
      <div class="grid-item">Item 2</div>
      <div class="grid-item">Item 3</div>
      <div class="grid-item">Item 4</div>
    </div>
    <!-- END COPY / PASTE -->