HTTP/2 changes HTTP transport framing and multiplexing; gzip and Brotli encode representation data. Enabling HTTP/2 does not automatically compress response bodies, and enabling gzip does not prove that a client negotiated HTTP/2. HTTP/2 header compression is another mechanism again. A reliable check records the negotiated protocol, response Content-Encoding, transferred body size, and the bytes obtained after decoding. ...
gzip is a response content coding: nginx can send a compressed representation that an accepting client decodes. Enabling it requires gzip on; gzip_types extends the eligible MIME types but does not activate compression by itself. The result depends on the response, request capabilities, and effective configuration. A smaller body is useful only if it still decodes to the intended content. ...
HTTP compression selects a coded representation, not a new media type. Gzipped text remains text after decompression. A useful check therefore asks three questions: was the coding acceptable, did the transfer become smaller, and did decoding recover the intended bytes? A compression checkbox or a successful status code cannot answer all three. ...