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

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

/* Global values */
row-gap: inherit;
row-gap: initial;
row-gap: revert;
row-gap: revert-layer;
row-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;
  row-gap: 20px;
}

#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>

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

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