grid-template-rows 行の高さを決める

<div class="frame">
  <span>上は中身の高さ</span>
  <span>下が残りの高さ</span>
</div>

.frame {
  display: grid;
  grid-template-rows: auto 1fr;
  height: 7rem;
  gap: 0.4rem;
}
.frame > span {
  background: #efeef8;
  padding: 0.4rem 0.65rem;
}