Hello Claude,
Dynamics GP 2013 has only been tested on Microsoft Windows Server 2008 R2 Enterprise or Standard Edition SP1 or later.
These are actually specified in the 'System Requirements for Microsoft Dynamics GP 2013' pages found in both CustomerSource or PartnerSource.
Because our GP Development hasn't actually tested Dynamics GP 2013 on Windows Server 2008 R2 Foundation, we can't guarantee that it will work completely, and technically it wouldn't be a supported environment.
Thank you,