You can do this by applying a filter to the binding. Select the gauge and in the binding section at the bottom of the screen, click the Add Filter button.
Give the filter a name and then define the filter body to add the text that you want to the value that is being displayed. In my example below, I wanted to create a filter for a temperature gauge so it showed the value with °C after and it looks like this:
return $filter('number')(value, 0) + " °C";
The (value,0) is controlling the number of decimal places, so if you wanted 1 decimal place it would be (value,1)
So when I look at the gauge, it looks like this:
Hope this helps.