Use Trim/Extend, Quick Connect, or Route and Fill to fill gaps in a fabrication model for ductwork, pipework, or electrical containment.
Video: Filling Gaps in a Fabrication Model in Revit
With any of these auto fill tools (Trim/Extend, Quick Connect, or Route and Fill), you can specify certain parts to be excluded. For more information, see Exclude Fabrication Parts from Auto Fill Tools.
To fill a gap between two straights, use the Trim/Extend tool. See Trim and Extend Elements.
To use the Quick Connect tool, follow these steps:
Quick connect is useful for stretching an offset or taper, or dragging a bend.

If more than one solution is available, Revit provides the option to open the Route and Fill editor.
To use the Route and Fill tool, follow these steps:
Edit panel
(Route and Fill).
The start point must be a connector on a fabrication part. However, the end point may be on a generic Revit element such as a duct, pipe, or mechanical equipment.
and
in the drawing area to view the possible solutions. Solutions requiring the fewest number of parts display first. As you click Next, the solutions displayed require more parts to complete the run.
The MEP Fabrication Parts palette displays all the parts used in the available solutions. A blue dot indicates that a part is used in the currently selected solution. As you position the cursor on a part in the list, the part is highlighted in the drawing area. In the MEP Fabrication Parts palette, select the check boxes to filter the list of available fittings and limit the number of solutions.
(Start Route from Opposite End).
(Cut Into). Smart Snapping is automatically disabled when Cut Into is enabled.
in the drawing area to accept a solution and complete the run.