Please note that the RadGauge is only the name of a suite that provides several gauge controls - RadRadialGauge, RadVerticalLinearGauge, RadHorizontalLinearGauge, RadSemicircleEastGauge, etc. Can you please double check for the above mentioned controls? I installed Q1 2015 version of our controls locally and it seems that the gauge controls are displayed correctly in the toolbox.
You can try to manually add the Telerik.Windows.Controls.DataVisualization assembly in the toolbox as described in the Adding UI for WPF to Visual Studio Toolbox
help article. If this does not help, can you tell me the version of Visual Studio and our controls which your are using? Also, keep in mind that you will need to open the XAML view in order for our controls to be displayed in the VS's Toolbox.
I hope this is useful.
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