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.
Part of the geda project, it's both free and open-source!
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!)