You're making hardware! You want to send out for PCBs, but you also want to make sure the files are right before sending - thus you need a Gerber Viewer. I use two viewers for my Gerber'in.
GC Prevue is not Open Source, but it is Free. You may need to download it every few months as it seems to 'expire'
Download it from http://www.graphicode.com/GC-Prevue#download
You'll need to type in a valid email address to download
Download and uncompress the installation zip
Once installed, you can run GC-Prevue and Import Files. One nice thing is you can import a full zip files of gerbs.
It can for-the-most-part autodetect what it is you're importing
With all the layers imported it can be a little difficult to see whats going on. Click on the files in the left side of the screen and hit h to 'hide' that layer. You can then drag-and-drop to reorder the layers so, say, you silkscreen is on top
You can also double-click on the files in the left pane to launch the explorer & change the layer colors and appearance
This tool is not as precise for viewing Gerbers, but it is great for proofing your PCB look, especially silkscreen, mask, and coloring. You can see the PCB in 3D which is handy for visualizing the design.
It's available from https://www.zofzpcb.com/
Once you've downloaded and installed it, you can run the Auto Import tool
And navigate to a folder with gerber files in it. If you use the default exporter, then Zofz will do its best to match the file types (last three letters) to determine what layer it is
And sort them into the proper layers and 'types'
it will then render it. You an click and drag to rotate the board around
There are various color-styles that are pre-progammed in, so you know what mask and silk will look like. or you can make your own (like my 'adafruit blue with ENIG' look above!)