Are you aware of the openFiles front-call
https://4js.com/online_documentation/fjs-fgl-manual-html/#fgl-topics/c_fgl_frontcall_standard_openfiles.html. (Note that is ...files plural
My inclination when transforming to use GBC is to precede FGL_GETFILE wth an explicit openFile(s) front-call as this gives you a little more control of what is rendered as opposed to FGL_GETFILE in isolation which will add its own UI if the file referenced has not been uploaded to the GAS temporary directory already.
Reuben