You can achieve the full height of the pie chart by setting the size attribute in the plotOptions of pie and removing margins, spacing and titles from the chart.

Code

    chart: {           

        margin: [0, 0, 0, 0],

        spacingTop: 0,

        spacingBottom: 0,

        spacingLeft: 0,

        spacingRight: 0

    },

    plotOptions: {

        pie: {

            size:'100%',

            dataLabels: {

                enabled: false

            }

        }

    }

Example (updated the fiddle to point to version 2.2.4)

Here is an example demonstrating this.

As to the linear gradients, I don't know if they have been implemented yet, but here is an example showing radial gradients.

Radial gradients are also part of Highcharts 3.0 now, here is an example

Update

Looks like as of Highcharts 3.0, this is not possible anymore due to the chart auto-scaling within the plot area, an excerpt from their documentation:

size: The diameter of the pie relative to the plot area. Can be a percentage or pixel value. Pixel values are given as integers. The default behaviour (as of 3.0) is to scale to the plot area and give room for data labels within the plot area. As a consequence, the size of the pie may vary when points are updated and data labels more around. In that case it is best to set a fixed value, for example 75%. Defaults to .

in my opinion this should not be the case since dataLabels are disabled. should probably be logged as a bug

Update (Aug 2014)

As per Torstein's comment, this is indeed still possible. slicedOffset needs to be set to 0 in addition to the margins begin set. (example)

$(function () {

    $('#container').highcharts({

        title: null,

        chart: {

            type: 'pie',

            margin: 0

        },

        

        plotOptions: {

            pie: {

                slicedOffset: 0,

                size: '100%',

                dataLabels: {

                    enabled: false

                }

            }

        },

        

        series: [{

            data: [

                ['Firefox',   44.2],

                ['IE7',       26.6],

                ['IE6',       20],

                ['Chrome',    3.1],

                ['Other',    5.4]

            ]

        }]

    });

});

#container {

    outline: 1px solid red;

    padding: 0;

}

<script src=https://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js></script>

<script src=https://code.highcharts.com/highcharts.js></script>

<div id=container style=height: 400px; width: 500></div>