ImageMagick is cross-platform command line utility that can batch convert images from one format to another. You can download the latest versions here.
Once installed, ensure that ImageMagick is available from the command line (ex. on Windows, add C:\path\to\ImageMagick to your Path). Then run:
magick mogrify -format gif *.webp
in the same folder as your .webp files (this can also be done from ex. a .bat file).
Note that you may need to use just mogrify -format gif *.webp without magick depending on the options you selected during the ImageMagick installation, your platform or if you are using an older version of ImageMagick.