<div></div>

div {
  position: relative;
  width: 200px;
  height: 60px;
  margin: 30px;
  border-radius: 30px;
  background: #fff;
  text-align: center;
}

div::before {
  content: "";
  position: absolute;
  top: -2px;
  bottom: -2px;
  left: -2px;
  right: -2px;
  background: linear-gradient(35deg, red, green);
  border-radius: 34px;
  z-index: -1;
}