Plugin licensing with third party server

I’m developing WordPress Plugin that I would like to distribute trough WordPress/Plugin directory site. As far as I understood, every plugin in WordPress plugin directory site needs to be GPL compatible and licensed under the GPL terms. Going through GPL license terms I figured out that all source code needs to be open to all users.
Now, I will try to explain how my plugin works. One of the plugin’s functionality is to communicate with my server providing data in XML file calculating “something” on server side and then returning result data to the site. I use this process of communication between site-server and my server due to the big database that is on my server that needs to be updated from time to time….. Client on his Site-server receives only short, calculated data from my server in XML file.
Since I would like that my plugin is also GPL licensed and provided trough WordPress Plugin directory I would need to know if It’s possible to license my plugin under GPL terms in form described before.