Gzip on Nginx seems not to work

[rethus]

Operating system: Ubuntu
OS version: 20.04
I’ve installed nginx and activated gzip globally in nginx.conf:

	gzip on;

	gzip_vary on;
	gzip_proxied any;
	gzip_comp_level 6;
	gzip_buffers 16 8k;
	gzip_http_version 1.1;
	gzip_types text/plain text/css application/json application/javascript text/xml application/xml application/xml+rss text/javascript;

I’ve restarted the nginx-server and do a check for a testpage:

PageSpeed Insights

There is a notice, that Text-Compression (or Content Compression) is not activated)

And if I have a look into the Network-Tab of the Dev-Console for the Page (https://demo2.kostenvoranschlag.expert), I see, that only a few files are shipped gziped:

Any Idea, what the issue here?