grid-template-columns 列の幅を決める

<div class="board">
  <span>狭い</span>
  <span>広い</span>
</div>

.board {
  display: grid;
  grid-template-columns: 1fr 2fr;
  gap: 0.5rem;
}
.board > span {
  background: #efeef8;
  padding: 0.55rem 0.65rem;
}