gap (grid-gap)

/* One <length> value */
gap: 20px;
gap: 1em;
gap: 3vmin;
gap: 0.5cm;

/* One <percentage> value */
gap: 16%;
gap: 100%;

/* Two <length> values */
gap: 20px 10px;
gap: 1em 0.5em;
gap: 3vmin 2vmax;
gap: 0.5cm 2mm;

/* One or two <percentage> values */
gap: 16% 100%;
gap: 21px 82%;

/* calc() values */
gap: calc(10% + 20px);
gap: calc(20px + 10%) calc(10% - 5px);

/* Global values */
gap: inherit;
gap: initial;
gap: revert;
gap: revert-layer;
gap: unset;

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

#flexbox {
  display: flex;
  flex-wrap: wrap;
  width: 300px;
  gap: 20px 5px;
}

#flexbox > div {
  border: 1px solid green;
  background-color: lime;
  flex: 1 1 auto;
  width: 100px;
  height: 50px;

}

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

#grid {
  display: grid;
  height: 200px;
  grid-template: repeat(3, 1fr) / repeat(3, 1fr);
  gap: 20px 5px;
}

#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 <code>gap</code> property.
  Don't you think that's fun and exciting? I sure do!
</p>

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