How to make a non-full circle stroke with gradient with a focus point?

I'm trying to make the element shown in the image below:

It is based on the Apple Watch widget for Air quality.

The circle is not fully round and the gradient changes across the line. Besides that, there is a 'point' that shows what the actual value is (I would like to set this with JavaScript).

I tried to make the half circle using dasharray and dashoffset. But the gradient I cannot get to cross the full line. I also cannot get the gap of the circle on the bottom without using transform: rotate on the object.

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<!-- language: lang-css -->

#Layer_1 {
   transform: rotate(135deg);
}
.ring {
   stroke-dasharray:314;
   stroke-dashoffset: 78;
   stroke-linecap: round;
}

<!-- language: lang-html -->

<svg version="1.1" id="Layer_1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" x="0px" y="0px" width="500px" height="500px" viewBox="0 0 120 120" enable-background="new 0 0 120 120" xml:space="preserve">

   <circle class="ring" fill="none" stroke="url(#linear)" stroke-width="10" stroke-miterlimit="10" cx="50%" cy="50%" r="50"/>

   <defs>
       <linearGradient id="linear" x1="0%" y1="0%" x2="100%" y2="0%">
           <stop offset="0%" stop-color="#05a"></stop>
           <stop offset="50%" stop-color="#a55"></stop>
           <stop offset="100%" stop-color="#0a5"></stop>
       </linearGradient>
   </defs>
</svg>

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While SVG content does not support conical gradients directly, you can combine a HTML element with a background-image: conic-gradient() and a SVG-defined mask that cuts out the arc and the position marker.

For the arc you can either use your technique with a circle and a stroke-dasharray. I find it easier to draw a path with an appropriate arc. The pathLength attribute will help in positioning along its length, but is not strictly needed for my solution.

There are multiple ways to achieve a progress marker, here I am going with a fake <animateMotion> animating a ring along the path that does not really move. The position is set as the keyPoints="x;x" attribute, with x a number between 0 and 1, to be set with Javascript.

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<!-- language: lang-js -->

const marker = document.querySelector('#progress animateMotion');

function position (x) {
  marker.setAttribute('keyPoints', `${x};${x}`);
}

position(0.2);

<!-- language: lang-css -->

#progress #arc {
  fill: none;
  stroke: white;
  stroke-width: 10;
  stroke-linecap: round;
}
#progress circle {
  fill: none;
  stroke: black;
  stroke-width: 4;
}

#meter {
  position: relative;
  width: 100px;
  height: 100px;
  background-image: conic-gradient(yellow, green, blue, red, orange, yellow);
  mask: url(#progress);
}

<!-- language: lang-html -->

<svg width="0" height="0">
  <mask id="progress" width="100" height="100">
    <path id="arc" d="M 25,85 A 40 40 0 1 1 75,85" pathLength="100" />
    <circle r="7">
      <animateMotion path="M 25,85 A 40 40 0 1 1 75,85"
                     keyTimes="0;1" begin="0s" fill="freeze"
                     keyPoints="0;0" />
    </circle>
  </mask>
</svg>
<div id="meter"></div>

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You are on the right track when it comes to the stroke dash array. In this example the circle and the focus point is made using a mask. The mask is applied to a group containing four different lines that in all makes the circular gradient. I know, the question is not so much about that gradient, but it kind of gets important now that SVG only has linear and radial gradients.

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<!-- language: lang-js -->

document.forms.form01.range.addEventListener('change', e => {
  document.getElementById('point').setAttribute('transform', `rotate(${e.target.value})`);
});

<!-- language: lang-css -->

body{
  background-color: #303030;
}

<!-- language: lang-html -->

<form name="form01">
  <input type="range" step="10" name="range" min="0" max="270" value="30">
</form>
<svg xmlns="http://www.w3.org/2000/svg" width="200" viewBox="0 0 100 100">
  <defs>
    <linearGradient id="lg1" x1="0" x2="0" y1="55" y2="0" gradientUnits="userSpaceOnUse">
      <stop offset="20%" stop-color="Gold"/>
      <stop offset="90%" stop-color="SpringGreen"/>
    </linearGradient>
    <linearGradient id="lg2" x1="0" x2="0" y1="55" y2="0" gradientUnits="userSpaceOnUse">
      <stop offset="20%" stop-color="DarkOrange"/>
      <stop offset="90%" stop-color="Gold"/>
    </linearGradient>
    <linearGradient id="lg3" x1="0" x2="0" y1="55" y2="0" gradientUnits="userSpaceOnUse">
      <stop offset="20%" stop-color="red"/>
      <stop offset="90%" stop-color="DarkOrange"/>
    </linearGradient>
    <linearGradient id="lg4" x1="0" x2="0" y1="55" y2="0" gradientUnits="userSpaceOnUse">
      <stop offset="20%" stop-color="DarkOrchid"/>
      <stop offset="90%" stop-color="red"/>
    </linearGradient>
    <mask id="m1">
      <circle r="45" stroke="white" stroke-width="10" stroke-linecap="round" stroke-dasharray="270 360" pathLength="360" />
      <g id="point" transform="rotate(30)">
        <circle transform="translate(45 0)" r="6" stroke="black" fill="white" stroke-width="3"/>
      </g>
    </mask>
  </defs>
  <g transform="translate(50 50) rotate(135)" stroke-linecap="round" stroke-width="25" mask="url(#m1)">
    <line transform="translate(50 0) rotate(30)" stroke="url(#lg1)" y2="55" />
    <line transform="rotate(70) translate(50 0) rotate(30)" stroke="url(#lg2)" y2="55" />
    <line transform="rotate(140) translate(50 0) rotate(30)" stroke="url(#lg3)" y2="55" />
    <line transform="rotate(210) translate(50 0) rotate(30)" stroke="url(#lg4)" y2="55" />
  </g>
</svg>

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