On the Host Configuration page of the Gateway Configuration Console, click the Gadget JS or Log JS download icons next to each configured server to get files appropriate for that server. The below link may help you further:
As you have created the gadgets by using IAC in PRPC6.3, the js files will be available. The existing PegaInternetApplicationComposer.js script still work for Pega7.2.2 if you are using the old <DIV> code.
As you mentioned the generate mashup code can generate under case type --> settings --> Integration --> Mashup.
When you use the Mashup code no need to add the PegaInternetApplicationComposer.js file, by default it will call the pzIncludeMashupScripts activity to generate the PegaInternetApplicationComposer.js file.