Modeling of gamma spectra is required in various fields. The growth of usage of spectrometric border monitoring methods to combat illicit trafficking of nuclear and other radioactive materials enhances the need for such techniques. With the development of the Gamma Spectrum Generator within the Nucleonica web-portal (www.nucleonica.net) a new ansatz is made, which allows authorized experts to perform spectrum modeling on a PC, wherever web access is available. The spectrum modeling toolkit (Gamma Spectrum Generator and easyMonteCarlo) must address various aspects, such as different detectors and their direct environment, various radioactive sources, their geometries and materials, distances and absorbers. After showing the accuracy of the modeling tools, more sophisticated scenarios can be dealt with. As a first example the nuclear security relevant masking issue is investigated experimentally and in extensive simulations using the described modeling tools.