Gaussian Tutorial: How to Build Large Molecules in Gaussview and Running Optimization. 

Using the Mouse in Gaussview:
To Rotate Molecule: Drag using LMB
To Zoom the Molecule: Drag using RMB
To Copy a Molecule in the Clipboard: Ctrl+C
To Paste the Molecule in the Drawing Window: LMB
To Substitute H with a Fragment From the Clipboard: Press LMB on the H atom
To Move the Molecule in the Window: Shift+LMB
To Move a Particular Molecule in a System of Multiple Molecules: Move the cursor to the particular Molecule and press Alt+Shift+LMB.