Thanks for your question. Gerber files do not have any information regarding Nets or layer thickness. however using FAB 3000 you can easily add those attributes and then export an ODB++.
1. Import your Gerber and Drill files into FAB 3000.
2. Assign the Layer Types (if needed) - required by ODB++ and for netlist extraction.
3. Using the FAB 3000 stackup editor, assign the following:
A. Correct Stackup order. You can drag layers up and down.
B. Add Dielectric Layers (if needed)
C. Assign the layer thickness for each. I just assigned random values for this tutorial move.
4. Extract a Netlist ( go to menu: Tools / Nets / Netlist Extraction...)
5. Export to ODB++.
Please view the 3 minute movie tutorial below (no sound):