gsim 0.20.1 for Mac could be downloaded from the developer's website when we last checked. We cannot confirm if there is a free download of this app available. The actual developer of this free Mac application is Vadim Zorin. Gsim for Mac lies within Education Tools, more precisely Science. We recommend checking the downloaded files with any free antivirus.
From the developer:
GSim is a small application for NMR spectroscopy. This is a free application for visualisation and processing of experimental and simulated nuclear magnetic resonance (NMR) spectra. The application has an interactive interface. It is very easy to use by any user.
You may want to check out more Mac applications, such as Arcade Math Blocks, Genealogy Pro or sciatrope, which might be similar to gsim.