Heroku push: gzip stdin not in gzip format
I my cases that I had this problem it seemed to be a Heroku problem. Just waiting 10 minutes or so did the trick for me.
After so many years and still, this issue occurs.Btw: My fix was to pin to a specific version as mentioned here:
heroku buildpacks:set https://github.com/heroku/heroku-buildpack-nodejs#v75 -a my-app
Same issue had occurred for my Java application which was built with Maven.It got fixed by configuring Java buildpack provided by Heroku (Earlier I was using custom buildpack which used to work on heroku for same application).