Chem3D: How do I bring a model from Gaussian into Chem3D Pro and vice versa?

Upgrade to the latest version of Chem3D Ultra (included in ChemOffice or Signals ChemDraw only). The latest version of Chem3D Ultra comes with a Gaussian client, which provides a direct interface with Gaussian.

Otherwise, to transfer files between Chem3D  and Gaussian you should use the .PDB file format.