I'm developing an app (for Android) which uses data received from my rest services. These services use also GPL libraries: if I have correctly understood, if a server side web application uses GPL libraries it has not requirements to release its source code as GPL. Has my app code any requirements if it uses this rest services?
You are most likely mistaken.
Your thinking revolves around typical web apps that use any old web browser, making the two pieces independent works.
Your situation most likely is actually a single work, with both pieces completely useless without the other. In that situation, the FSF maintains it all is a single work with the Android piece also subject to the requirements of the GPL.
That is the FSF's position, which to my knowledge hasn't been tested in court. Ignore at your own peril.