With GerbView you can easily convert your ODB++ projects to Gerber or Extended Gerber file format.
For this conversion to work successfully you will need to do the following steps:

Step 1:
Load your ODB++ file.
You can load the ODB++ file using either Open Project command or the Add Layer command.
If the project contains more than one step, the following dialog will be displayed:

Select the step you want to open, and all layers related the selected step will be loaded from the ODB++ project, and then displayed as shown below:

Step 2:
Goto the tab named Conversion and locate the button named Save as RS-274X. When you press this button, you will notice that it has two options. To create multiple Gerber files, you will have to select the section option, name Save to Multiple Gerber Files. Each selected ODB++ layer will then be converted to a separate Gerber file.

A new dialog will be displayed where each layer is listed:

Step 3:
Select the layers that you want to convert to Gerber. By default, all layers will initially be selected.
Make sure that Gerber RS-274X is the selected output format and select an existing folder for converted Gerber files.
You may also change the extension to use for the Gerber files if you wish.
When you're satisfied with your selection you press the Start button.
The conversion will then start, after the process is complete you will see a message in the output window at the bottom of the screen.
You can then open the output folder and see the converted Gerber files:

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