// HTML -------------->

<button class="mouse-cursor-gradient-tracking">
  <span>Hover me</span>
</button>

// CSS -------------->

.mouse-cursor-gradient-tracking {
  position: relative;
  background: #2379f7;
  padding: 0.5rem 1rem;
  font-size: 1.2rem;
  border: none;
  color: white;
  cursor: pointer;
  outline: none;
  overflow: hidden;
}
.mouse-cursor-gradient-tracking span {
  position: relative;
}
.mouse-cursor-gradient-tracking::before {
  --size: 0;
  content: '';
  position: absolute;
  left: var(--x);
  top: var(--y);
  width: var(--size);
  height: var(--size);
  background: radial-gradient(circle closest-side, pink, transparent);
  transform: translate(-50%, -50%);
  transition: width .2s ease, height .2s ease;
}
.mouse-cursor-gradient-tracking:hover::before {
  --size: 200px;
}
JavaScript
var btn = document.querySelector('.mouse-cursor-gradient-tracking')
btn.onmousemove = function (e) {
  var x = e.pageX - btn.offsetLeft
  var y = e.pageY - btn.offsetTop
  btn.style.setProperty('--x', x + 'px')
  btn.style.setProperty('--y', y + 'px')
}

// JS -------------->

var btn = document.querySelector('.mouse-cursor-gradient-tracking')
btn.onmousemove = function (e) {
  var x = e.pageX - btn.offsetLeft
  var y = e.pageY - btn.offsetTop
  btn.style.setProperty('--x', x + 'px')
  btn.style.setProperty('--y', y + 'px')
}