The most straight way to disable magic_quotes_gpc on your dedicated server is to do it in the php.ini however it is not the case with DirectAdmin.
Even if you disable it in php.ini it will get commented out after you recompile php. And since it is commented out, by default it is enabled. 9You can find the official position of DirectAdmin re this topic here)
So, you will have to disable magic_quotes_gpc directly through .htaccess file for each of your domains manually.
To do it, place the .htaccess file in the root folder of the domain (i.e. public_html) and place the following code on a separate line:
php_value magic_quotes_gpc off
This will turn magic quotes off for this articular domain.