column-gap (grid-column-gap)

/* キーワード値 */
column-gap: normal;

/* <length> 値 */
column-gap: 3px;
column-gap: 2.5em;

/* <percentage> 値 */
column-gap: 3%;

/* グローバル値 */
column-gap: inherit;
column-gap: initial;
column-gap: unset;

<div id="flexbox">
  <div></div>
  <div></div>
  <div></div>
</div>

#flexbox {
  display: flex;
  height: 100px;
  column-gap: 20px;
}

#flexbox > div {
  border: 1px solid green;
  background-color: lime;
  flex: auto;
}

<div id="grid">
  <div></div>
  <div></div>
  <div></div>
</div>

#grid {
  display: grid;
  height: 100px;
  grid-template-columns: repeat(3, 1fr);
  grid-template-rows: 100px;
  column-gap: 20px;
}

#grid > div {
  border: 1px solid green;
  background-color: lime;
}

<p class="content-box">
  This is some multi-column text with a 40px column
  gap created with the CSS `column-gap` property.
  Don't you think that's fun and exciting? I sure do!
</p>

.content-box {
  column-count: 3;
  column-gap: 40px;
}