As commented by Paul LeBeau, you need to convert the wavy shape to one path, then you can fill the wavy shape with a linear gradient as shown in this example:
<svg viewbox="7.5 0 60 10">
<defs>
<linearGradient id="gradient">
<stop offset="5%" stop-color="#FFC338" />
<stop offset="95%" stop-color="#FFEA68" />
</linearGradient>
</defs>
<path fill="url(#gradient)" d="M0 10 V5 Q2.5 2.5 5 5 t5 0 t5 0 t5 0 t5 0 t5 0 t5 0 t5 0 t5 0 t5 0 t5 0 t5 0 t5 0 t5 0 t5 0 V10" />
</svg>