Dark red to dark plum to marine gradient palette editor

/* https://grabient.com/_gEAgs9e0VgElgtZhRbgIYgBlgBlgPFgHMgOG */

background: linear-gradient(90deg, #8a1000 0.000%, #880100 16.667%, #700000 33.333%, #490023 50.000%, #200445 66.667%, #021559 83.333%, #002f5e 100.000%);

<svg xmlns="http://www.w3.org/2000/svg" width="800" height="400" viewBox="0 0 800 400" preserveAspectRatio="none">
            <!-- https://grabient.com/_gEAgs9e0VgElgtZhRbgIYgBlgBlgPFgHMgOG -->
            <defs>
              <linearGradient id="gradient" x1="0.000" y1="0.500" x2="1.000" y2="0.500">
        <stop offset="0.000" stop-color="#8a1000" /><stop offset="0.167" stop-color="#880100" /><stop offset="0.333" stop-color="#700000" /><stop offset="0.500" stop-color="#490023" /><stop offset="0.667" stop-color="#200445" /><stop offset="0.833" stop-color="#021559" /><stop offset="1.000" stop-color="#002f5e" />
              </linearGradient>
            </defs>
            <rect x="0" y="0" width="800" height="400" fill="url(#gradient)" />
          </svg>

["#8a1000", "#880100", "#700000", "#490023", "#200445", "#021559", "#002f5e"]

// https://grabient.com/_gEAgs9e0VgElgtZhRbgIYgBlgBlgPFgHMgOG
// cosine gradient palette — https://iquilezles.org/articles/palettes/
const float STEPS = 7.0;
const float ANGLE = 90.0;

vec3 palette(float t) {
    vec3 a = vec3(0.256, 2.877, -4.843);
    vec3 b = vec3(0.293, 2.905, 5.211);
    vec3 c = vec3(0.536, 0.101, 0.101);
    vec3 d = vec3(0.965, 0.460, 0.902);
    return a + b * cos(6.283185 * (c * t + d));
}

// N stops sampled from the palette, blended like the CSS gradient
vec3 stops(float t) {
    float n = STEPS - 1.0;
    float x = clamp(t, 0.0, 1.0) * n;
    return mix(palette(floor(x) / n), palette(min(floor(x) + 1.0, n) / n), fract(x));
}

void mainImage(out vec4 fragColor, in vec2 fragCoord) {
    vec2 p = fragCoord - 0.5 * iResolution.xy;
    // CSS angle: 0 = up, clockwise; span = CSS gradient-line length
    vec2 dir = vec2(sin(radians(ANGLE)), cos(radians(ANGLE)));
    float len = abs(iResolution.x * dir.x) + abs(iResolution.y * dir.y);
    float t = dot(p, dir) / len + 0.5;
    fragColor = vec4(stops(t), 1.0);
}

[
  [0.256, 2.877, -4.843],
  [0.293, 2.905, 5.211],
  [0.536, 0.101, 0.101],
  [0.965, 0.460, 0.902]
]