LogFormat "%h %l %u %t \"%r\" %>s %b mod_gzip:%{mod_gzip_result}n In:%{mod_gzip_input_size}nOut:%{mod_gzip_output_size}n:%{mod_gzip_compression_ratio}npct." full_plus_gzip
[17:29:14]
CustomLog /var/log/httpd/mod_gzip.log full_plus_gzip

Example of log entry:
/reviews/cooling/HSR2_4.html HTTP/1.1" 200 6289 mod_gzip: DECHUNK:OK In:23352 Out:6289:74pct.