CSS Gradient Animation — https://www.gradient-animator.com/

granimation.css

background: linear-gradient(270deg, #430fe4, #234ae8, #15a32d, #f55519, #d71abf);
background-size: 1000% 1000%;
-webkit-animation: AnimationName 6s ease infinite;
-moz-animation: AnimationName 6s ease infinite;
-o-animation: AnimationName 6s ease infinite;
animation: AnimationName 6s ease infinite;
@-webkit-keyframes AnimationName {
    0%{background-position:0% 50%}
    50%{background-position:100% 50%}
    100%{background-position:0% 50%}
}
@-moz-keyframes AnimationName {
    0%{background-position:0% 50%}
    50%{background-position:100% 50%}
    100%{background-position:0% 50%}
}
@-o-keyframes AnimationName {
    0%{background-position:0% 50%}
    50%{background-position:100% 50%}
    100%{background-position:0% 50%}
}
@keyframes AnimationName {
    0%{background-position:0% 50%}
    50%{background-position:100% 50%}
    100%{background-position:0% 50%}
}