This module says it’s used with nodejs backend. I don’t think it’s for browsers.
A nodejs module for integration of your backend with the payment services provided by Gerencianet.
Add custom nodejs app : How Add nodejs App to frappe & bench? - #5 by revant_one
Add custom js for browser: Build.json and custom CSS (use the pre built *.min.{js,css} by third party, or feed js dist output of any bundler to build.json)