That module doesn’t have any browser-compatible file included in it, so the best option is to use bundle.run:
gff = require('https://bundle.run/@gmod/gff')
You can try out the module debugger that can identify which patterns work.
For that .gz file, it should work - though be sure to confirm that the server you’re loading the file from supports CORS and is https. Loading a file from FTP, for example, won’t work directly, because of basic security limitations on the browser level.