This page has all the settings specific to a Heat map.
Choose a Colour Gradient from the list of available options
Intensity indicates the intensity of each heat point.
Radius is the radius to draw each data point on the map.
Unit is the distance units of the radius.
When the unit is set to pixels the size of each data point will always be the same radius, regardless of zoom level. When set to meters (sic), the size of the data points will scale based on zoom level so as to ensure that the radius is spatially accurate.