After trying several options for viewing Gerber files for PCB layouts I settled on gerbv. It’s open source, free, and does not require any registration.
The Windows installer and source code are available here. If you’re using Linux you can either download the source code and build it yourself or check your distribution’s add/remove software repository. I don’t know what is involved in getting it running on a Mac.
When gerbv starts you will see a window similar to Figure 1.
Your first instinct is going to be to click on the folder icon in the toolbar to load your Gerber files. Don’t. That function is for loading a previously saved gerbv project. If you have the application running, hover over the icons to see the tooltips that tell you what each one does.
Notice there is a button with a large PLUS (+) sign on it in the lower left corner of the gerbv window. Pressing that button will bring forth gerbv’s file dialog. Navigate into the folder containing your Gerber files and, using [ctrl] and/or [shift], select multiple files to view and click the OPEN button. Gerbv does not care about the extensions as long as the files contain valid gerber data.
Once your files are loaded gerbv should look similar to Figure 2.
Here’s a few things you might want to play with (from the Main menu):
Tools -> Measure Tool
Note the check boxes next to each layer in the leftmost pane. Use those to toggle individual layers on and off.
Viewing your Gerber plots prior to sending them for fabrication can often uncover problems that were missed while doing the PCB layout. Better that you find any issues now than to wait for your boards to come back from fab.