Sempre que eu mudo a tag do float, não fica centralizado na forma horizontal, apenas vertical.
 
	Sou iniciante ainda, sorry kkk
 
	 
 
<div class="topnav">
  <!--<a href="#"><img src="nr-logo.jpg" width="50"></a>-->
  <a href="#">Inicio</a>
  <a href="#">MENU 1</a>
  <a href="#">MENU 2</a>
  <a href="#">MENU 3</a>
  <a href="#">MENU 4's</a>
  <a href="#">MENU 5</a>
  <a href="#">MENU 6</a>
</div>
.topnav {
  overflow: hidden;
  background-color: #fff;
  border-style: outset;
  border-width: 0.1px;
  text-align: justify;
}
.topnav a {
  float: left;
  display: block;
  color: #000;
  padding: 15px 30px;
  text-decoration: none;
  font-weight: normal;
  text-align: justify;
}
.topnav a:hover {
  background: -webkit-linear-gradient(#21d4fd, #b721ff, #21d4fd, #b721ff);
  background: -o-linear-gradient(right, #21d4fd, #b721ff, #21d4fd, #b721ff);
  background: -moz-linear-gradient(right, #21d4fd, #b721ff, #21d4fd, #b721ff);
  background: linear-gradient(right, #21d4fd, #b721ff, #21d4fd, #b721ff);
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  color: black;
}