Returns the DIMGAP value of this object.

DIMGAP specifies the gap between dimension text and dimension lines. 

A negative value for DIMGAP draws a reference box round the dimension text. 

Text is placed inside a dimension line only if there will be two line segments each as long as abs(DIMGAP). 

The value from the dimension style of this object will be returned unless that value has been overridden, in which case, the override value will be returned.