it is passed a bit since I posted my question and I came back again to solve this issue.
At the following path: /opt/bitnami/apache2/, I have the following folder and files:
On httpd.conf file there is a call for the deflate.conf file through the "Include conf/deflate.conf" instruction. So I inserted my directives for the gzip compression inside the deflate.conf file.
This was in agreement with the reply coming from the same question that I posted on stackoverflow.com some month ago:
Once I did this change, I restarted apache but without effects. How can I do this?