...select the Dimension of interest and edit the text (note that you will get a 'Warning' that you're creating a Dimension with "manual text").
As for adding a 'gap' to a witness line, go to...
Insert -> Symbol -> User Defined...
...set the 'Use Symbols in:' option to 'Utility Directory' and then scroll down the list until you find a series of items labeled 'GAPxxx' (the 'xxx' is the size of the 'GAP' in inches). Select the desired one and then with the 'Add to Drafting Object' button active, select the Dimension that you wish to add the 'GAP' to its witness line and follow the steps described on the Queue/Status line.
John R. Baker, P.E.
Product 'Evangelist'
Product Design Solutions
Siemens PLM Software Inc.
Industry Sector
Cypress, CA