Nginx can compress a [i]response[/i] with gzip, like Apache's mod_deflate SetOutputFilter DEFLATE, but it doesn't seem to have the ability to SetInputFilter DEFLATE, i.e. to accept a gzip-compressed [i]request[/i] and gunzip it before any further processing occurs (for example before it's passed along to a script via extensions such as FastCGI or Phusion Passenger).
Ronen