Ruby 1.9.3 を使用している場合、File.read が存在しない場合は次のようにします。
if cgi.request_method == 'GET'
print File.open('path/to/file.json').read
end
また、http://www.ruby-doc.org/stdlib-1.9.3/libdoc/cgi/rdoc/CGI.html#method-i-outを使用する必要があります
cgi.out("nph" => true,
"status" => "OK", # == "200 OK"
"server" => ENV['SERVER_SOFTWARE'],
"connection" => "close",
"type" => "text/html",
"charset" => "iso-2022-jp",
# Content-Type: text/html; charset=iso-2022-jp
"language" => "ja",
"expires" => Time.now + (3600 * 24 * 30),
"cookie" => [cookie1, cookie2],
"my_header1" => "my_value",
"my_header2" => "my_value") { "string" }