By default, Firebase Hosting already gzips all your files. Here, for example, there are response headers for the css file that I posted on firebase. Check out the Content-Encoding header:
Accept-Ranges:bytes Cache-Control:max-age=7178000 Connection:keep-alive Content-Encoding:gzip Content-Length:3483 Content-Type:text/css; charset=utf-8 Date:Sun, 10 Jan 2016 02:09:57 GMT ETag:"4c94283e07340e9cc0237fc2a349c94d" Last-Modified:Sun, 10 Jan 2016 00:10:31 GMT Server:nginx Strict-Transport-Security:max-age=31556926; includeSubDomains; preload Vary:Accept-Encoding Via:1.1 varnish X-Cache:HIT X-Cache-Hits:1 X-Powered-By:Express X-Served-By:cache-lax1432-LAX
source share