<h2>Auto-fit</h2>
<div class="auto-fit">
  <div class="child">A</div>
  <div class="child">B</div>
  <div class="child">C</div>
  <div class="child">D</div>
</div>
<h2>Auto-fill</h2>
<div class="auto-fill">
  <div class="child">A</div>
  <div class="child">B</div>
  <div class="child">C</div>
  <div class="child">D</div>
</div>

.auto-fit {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(100px, 1fr));
  grid-gap: 1em;
}

.auto-fill {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(100px, 1fr));
  grid-gap: 1em;
}

/* for demo purpos */
.child {
  background: pink;
  min-height: 100px;
  display: flex;
  justify-content: center;
  align-items: center;
  font-size: 3rem;
}