¸ñÁÐ϶ | grid-column-gap

/* <length> values */
grid-column-gap: 20px;
grid-column-gap: 1em;
grid-column-gap: 3vmin;
grid-column-gap: 0.5cm;

/* <percentage> value */
grid-column-gap: 10%;

/* Global values */
grid-column-gap: inherit;
grid-column-gap: initial;
grid-column-gap: unset;

<length-percentage>where 
<length-percentage> = <length> | <percentage>

<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;
  grid-column-gap: 20px;
}

#grid > div {
  background-color: lime;
}