The Auto Fit Labels option lets you toggle between "auto fitting" the labels to the space available on the gauge and using fixed sizes for the Label text. By default, the labels displayed on a gauge "auto fit" to the space available using the font settings in the determine the relative proportions of the auto-fitted labels. When Auto Fit is off, the font sizes are determined using the Gauge Labels settings only.
Enabling and disabling Auto Fit Labels
In Present, you can enable and disable Auto Fit Labels on the right-click menu, under Visual Options:
Tip: You might want to disable Auto Fit Labels in Present if your Gauges all have a different appearance to one another. In this case, you can set Auto Fit Labels Off, ensuring that the settings for the Gauge Labels from the Formatting panel are used consistently and are not re-sized.