Zoom up on the drawing view where you wish to add the gap so that you can see what you doing.
Then go to Insert -> Symbol -> User Defined Synbol... and when the dialog comes up under the item titled 'Use Symbols in:' select 'Utility Directory'.
When the list of symbols come up scroll down till you get to a series of 3 named 'GAP06', 'GAP125' and 'GAP25'. The larger the number of the larger the gap so you may have to experiment a bit (note that even though the numbers appear to correlate to Imperial units (Inches) there results will be the same in English or Metric drawings.
Now go the bottom of the dialog and you will find 4 icons, you want to select the one on the far left, labeled 'Add to Drafting Object'.
Now select the dimension that you wish to add the 'gap' to (you can pick it anywhere just as long as it highlights).
Once the dimension is highlighted now place your cursor just a little below where you wish the gap to be created (actually this will be the bottom of the gap if the extension line is vertical) and select.
Now you have a gap. Repeat as needed.
And before you ask, NO, they are not associative so if you move the dimension and the gap needs to be moved, you'll have to delete the current one and create a new one.
I know this sounds old and archaic, but that's only because it is.
John R. Baker, P.E.
Product 'Evangelist'
NX Design
Siemens PLM Software Inc.
Cypress, CA