justify-items マスの中で子を左右に寄せる

<div class="board">
  <span>A</span>
  <span>B</span>
</div>

.board {
  display: grid;
  grid-template-columns: 1fr 1fr;
  justify-items: center;
  gap: 0.4rem;
  background: #f3f1ec;
  padding: 0.5rem;
}
.board > span {
  background: #efeef8;
  padding: 0.3rem 0.6rem;
}