I am not sure what do you mean by 'gradually fill effect', but I think that you may wonder about an initial animation on your chart.
You can change the function responsible for making your gradient a little bit to achieve the chart with an initial animation. I am sending you this chart below:
http://jsfiddle.net/td2v4u4z/31/
If you think that is a good idea to have a functionality of simple adding similar gradient to your gauge, I think that you can ask for this feature on Highcharts uservoice. The best ideas (with biggest number of votes) gets to be implemented in future Highcharts versions.
https://highcharts.uservoice.com/
In case of the second question you have asked, I think that you should be able to add custom function for rotating your dataLabels, so they will be inside your donut chart:
rotate = function() {
$.each(options.series, function(i, p) {
angle1 = 0;
angle2 = 0;
angle3 = 0;
allY = 0;
$.each(p.data, function(i, p) {
allY += p.y;
});
$.each(p.data, function(i, p) {
p.dataLabels = p.dataLabels || {};
angle2 = angle1 + p.y * 360 / (allY);
angle3 = angle2 - p.y * 360 / (2 * allY);
if (angle3 >= 180) {
p.dataLabels.rotation = 0 + angle3;
} else {
p.dataLabels.rotation = 0 + angle3;
}
angle1 = angle2;
});
});
};
I have made simple example how it can work: http://jsfiddle.net/j7as86gh/31/
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