Hi Rene,
It is very important to pop-up a new window, since it is an external application that is being called, which cannot be shown a custom tab of the GAD applicatiion. The OTP is just something that is obtained from DB ( calling the stored procedure ), but in the end it is used to construct an URL, which is being called by the pop-up window to reach the external application. So the final URL loaded in the pop-up is something: http://remote_server/uri&user=[genesys_user]&OTP=[obtained_otp_by_calling_stored_procedure_in_the_db].
Anyway I have solved it, but I haven't been able to find the exact event to trigger my custom jsp. The javascript event triggering my jsp is still <javascript-onaddinteraction> ( which is similar to: as soon as the call enters GAD ), but in the end I have used this.
So if anyone has another event which I can link my jsp execution, I would gladly use it. Anyone who knows which javascript can describe better: when call was answered by agent, I will be very happy to know it.... I am still here waiting for an answer....

Everything reduces to a jsp doing a database interrogation, but with one small thing that you need to know, unless you are ready to digg for hours

. How to load a custom database driver in the Genesys Desktop Tomcat distribution and let Tomcat DBCP classloader load it? Well it is easy: put your driver (ojdbc6.jar, mysql-connector-5..x.x.jar etc. ) into $CATALINA_HOME/lib folder, but modify the $CATALINA_HOME/bin/GDesktopStarter.ini and add the appropriate line to this jar file in the .INI ( you'll figure it out once you open the file ), otherwise it will not be loaded just by dropping the jar in the folder.
Regards,
Valeriu