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Gerber is an open-source online Gerber viewer. Online Gerber Viewer is another online platform where you can view Gerber files.
Using File viewer, the data for copper, solder mask, silkscreen, drill, etc., layers can be viewed, measured. Visually, the data can be viewed. DFM /DRC operations cannot be performed. If you need the Gerber viewer only for the above purpose then any freely available Gerber file viewers can be used. Viewmate is a good free Gerber file viewer.
A viewer is simply that: something to view your Gerber plots. They don't check spacing, design rules, etc. (at least, not the free ones I've used). Most PCB layout tools have built-in Design-rule check.
However, there ARE tools out there that do perform DFM, if THAT's actually what you're looking for, as opposed to a "viewer". They are used by PCB manufacturers, and they're not cheap.
What are you REALLY looking for beyond what your layout tool provides?