nav {
     background: lightblue;
     color: white;
     display: block;
     height: 35px;
     line-height: 35px;
     padding: 0px;
     text-decoration: none;
}

nav ul, nav li {
    list-style-type:none;
     margin : 0px auto;
}

.gnav {
  display: flex;
  height: 2rem;
  
  /* margin: 0 auto; */
  width: 100%;
}
.gnav > li {/*親階層のみ幅を25%にする*/
  width: 25%;
}
/*全てのリスト・リンク共通*/
.gnav li {
  /* list-style: none; */
  position: relative;
}
.gnav li a {
  background:lightblue;
  border-right: 1px solid #eee;
  color: darkblue;
  display: block;
  height: 2rem;
  line-height: 2rem;
  text-align: center;
  text-decoration: none;
  width: 100%;
}

/*子階層以降共通*/
.gnav li li {
    height: 0;
    overflow: hidden;
    transition: .5s;
}
.gnav li li a {
    border-top: 1px solid #eee;
}
.gnav li:hover > ul > li {
    height: 2rem;
    overflow: visible;