Use a plot band instead of a plotline and define color as linear gradient:
chart.yAxis[0].addPlotBand({
color: {
linearGradient: {
x1: 0,
x2: 0,
y1: 0,
y2: 1
},
stops: [
[0, '#696ec7'],
[1, '#c4c4c4']
]
},
from: 0,
to: 700
});
Live demo: http://jsfiddle.net/BlackLabel/w0hy3z6e/
Docs: https://www.highcharts.com/docs/chart-design-and-style/colors