If your software only interacts with the server through the standard OS interfaces or other standardized interfaces, then you can use the GPL server without question.
The GPL license applies to entire applications, but there are limits to what is considered part of such an application. In particular, the boundaries are:
- system APIs
- API's that conform to well known standards
- separate processes that communicate "at arms length". At arms length essentially means here that the processes communicate through a standard interface without knowing about each others internals.
It essentially means that if your software communicates through a defined interface to the server, where that interface is not specific to that particular server, then the server's license doesn't carry over to your application.