• フレックス：https://caniuse.com/mdn-css_properties_row-gap_flex_context
• グリッド：https://caniuse.com/mdn-css_properties_row-gap_grid_context

フレックス、グリッド間の行方向の隙間の幅を指定します。

.flex {
  display: flex;
  flex-wrap: wrap;
  row-gap: 1rem;
}
.flex > * {
  flex: 1;
  flex-basis: 30%;
  background-color: #ddd;
  border: 1px solid #ccc;
  padding: .5rem;
}

<div class="flex">
 <div>Flex#1</div>
 <div>Flex#2</div>
 <div>Flex#3</div>
 <div>Flex#4</div>
 <div>Flex#5</div>
 <div>Flex#6</div>
</div>

.grid {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  row-gap: 1rem;
}
.grid > * {
  background-color: #ddd;
  border: 1px solid #ccc;
  padding: .5rem;
}

<div class="grid">
  <div>Grid#1</div>
  <div>Grid#2</div>
  <div>Grid#3</div>
  <div>Grid#4</div>
  <div>Grid#5</div>
  <div>Grid#6</div>
</div>

• https://drafts.csswg.org/css-align/#column-row-gap
• https://developer.mozilla.org/ja/docs/Web/CSS/row-gap
• https://caniuse.com/mdn-css_properties_row-gap_grid_context