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Application running over Debian and GPL

If i develop a application (in c or php) and install on Debian (GPL license) and distribuite this solution (my app + debian) how a appliance (both installed in a hardware) and sell this appliance (harware + debian + my app). The my app is necessarily a app GPL and i need provide the source code to my client or i can keep my app proprietary and not provide the source code from my app?

In this environment no provide the source code is a violation from GPL?