I'm not sure about how the svg would work here (though it might work fine), but you can do that with just CSS as well:

@keyframes tab-gradient-anim{
  from{ background-position-x: 0% }
  to{ background-position-x: 200% }
}
.tab-content[selected]{
  background-image: linear-gradient(
        90deg,
        rgb(255, 0, 0) 0%,
        rgb(255, 154, 0) 12%,
        rgb(208, 222, 33) 24%,
        rgb(79, 220, 74) 35%,
        rgb(63, 218, 216) 44%,
        rgb(47, 201, 226) 50%,
        rgb(28, 127, 238) 60%,
        rgb(95, 21, 242) 70%,
        rgb(186, 12, 248) 80%,
        rgb(251, 7, 217) 90%,
        rgb(255, 0, 0) 100%
    );
  background-repeat: repeat-x;
  background-size: 200% 100%;
  animation: tab-gradient-anim 2s infinite linear;
  background-clip: text;
  color: transparent;
}