I'm planning to code a mobile app that uses an opensource server liscenced in GPL. So my app will make webservices call to that server.
Will I be obliged to publish the code of my mobile application?
Not at all. Calling a web service will unlikely ever be considered as either copying or creating some derivative work of the web service you call. This is contrast if you were to run and call GPL-licensed code in the same process from your mobile application.