The nginx.sample.conf currently shipped by phpBB includes the following:

http {

    # Compression - requires gzip and gzip static modules.

    gzip on;

    gzip_static on;

    gzip_vary on;

    gzip_http_version 1.1;

    gzip_min_length 700;

    # Compression levels over 6 do not give an appreciable improvement

    # in compression ratio, but take more resources.

    gzip_comp_level 6;

    # IE 6 and lower do not support gzip with Vary correctly.

    gzip_disable "msie6";

    # Before nginx 0.7.63:

    #gzip_disable "MSIE [1-6]\.";

and has the the following on top:

# Global settings have been removed, copy them

# from your system's nginx.conf.

which is basically shows the issue where this request is originating from:

The most common use case is to have such http{} blocks in the global nginx.conf and only include additional configurations (for e.g. different vhosts) from the nginx.conf with something like e.g.:

        include /etc/nginx/conf.d/*.conf;

        include /etc/nginx/sites-enabled/*;

Based on the explanation above i would suggest to remove the surrounding http{} block in the nginx.sample.conf including the gzip stuff (which should be either in the nginx.conf or in a separate gzip.conf and included in the nginx.conf). Especially the gzip code is also something which is not directly related to phpBB.