I am not an attorney, but my understanding is that anyone can modify code released under the GPL. If you do not release your server-side code and do not include it in the GPL, then you would not have to reveal it.
I would think that a clear disclaimer regarding the use of separate proprietary code that is not part of the GPL would cover the issue.
GNU General Public License v2.0 - GNU Project - Free Software Foundation (FSF)
Most of the GPL focuses on how 3rd parties are allowed to copy, distribute and modify the code that you do release.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin