GDS files are binary files. There is no human readable representation of that raw data. The only option is to use any of the GDSII importers/readers, like ADS or Klayout as suggested by others, to view the layout data the file contains. This can be compared to picture files like JPG files. You can open and view the contents in a suitable viewer/editor program but not directly read the raw data in the file itself.
Gerber files are mostly text files so it is possible to open and read those in simple text editors like Notepad/Notepad++ etc.