GramJS uses webpack to provide a browser file if needed. You can also import it into your project and it will compile fine.
To get a JS file first clone the repo and install all dev dependencies then simply use
$ webpack
you will find the bundle file in the browser folder.
Currently, bundle files aren't provided separatly and need to be compiled. GramJS has a telegram group where you can ask for a file if building is out of the option for you