The advantage of ODB++ compared to Gerber is the following:
1. Layer Types and Stackup are already defined.
2. Netlist and Components are defined.
Thus after importing an ODB++ you may immediately run DFM Check without any further setup.
Gerber data is just a text file containing X,Y coordinates with aperture shapes (i.e circle, rectangle, thermal, etc.). This means to run a DFM Check on Gerbers requires the following additional steps:
1. Assign Layer Types and Stackup.
2. Extract a Netlist containig electrical connectivity.
3. Optional: Extract a border layer and any cutouts.
4. Optional: Generate components from Paste and Silkscreen layers.
Here's a tutorial movie showing how to perform those steps above, https://support.numericalinnovations.com/support/discussions/topics/14000012621
Lastly, FAB 3000 can run as a command-line tool, so you can automate all the steps above using the import matrix file, plus run DFM Checks simultaneously. https://www.numericalinnovations.com/pages/fab-3000-matrix-file-description
Thanks again, and please let me know if you have any further questions.