@property --rotate {
  syntax: '<angle>';
  initial-value: 0deg;
  inherits: false;
}
.gradient-button {
  animation: rotate-gradient 1s linear infinite;
  background-image: linear-gradient(var(--rotate), red, yellow, green);
}
@keyframes rotate-gradient {
  from {
    --rotate: 0deg;
  }
  to {
    --rotate: 360deg;
  }
}

<button id="fill" class="gradient-button">Fill Form</button>