Можешь только градиент в html засунуть
<body>
<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" aria-hidden="true" focusable="false" style="width:0;height:0;position:absolute;">
<defs>
<linearGradient id="fpaint0_linear" x1=".055" y1="32.033" x2="55.946" y2="32.033" gradientUnits="userSpaceOnUse">
<stop stop-color="#944DEB" />
<stop offset="1" stop-color="#4F76DC" />
</linearGradient>
</defs>
</svg>
<svg class="icon icon-garant" fill="url(#fpaint0_linear)">
<use href="spriteraw.svg#garant" ></use>
</svg>
</body>
а в спрайте убрать fill
<?xml version="1.0" encoding="utf-8"?>
<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
<symbol viewBox="0 0 56 64" id="garant" xmlns="http://www.w3.org/2000/svg">
<path fill-rule="evenodd" clip-rule="evenodd" d="M28.888.215l25.575 ..."/>
</symbol>
</svg>
