Quoted from: White, Patrick. "Earth Global Reference Atmospheric Model (GRAM) Overview and Future Improvements." 42nd COSPAR Scientific Assembly 42 (2018): C4-2. https://ui.adsabs.harvard.edu/abs/2018cosp...42E3651W/abstract 

NASA Marshall Space Flight Center's (MSFC) Earth Global Reference Atmospheric Model (GRAM) is a reference or standard atmosphere used for design and mission planning of various aerospace systems. Earth-GRAM was developed in response to the need for a design reference atmosphere that provides complete global geographical variability, complete altitude coverage (surface to orbital altitudes), and complete seasonal and monthly variability of thermodynamic variables and wind components. A unique feature of GRAM is that, in addition to providing the geographical, height, and monthly variation of the mean atmospheric state, it includes the ability to simulate spatial and temporal perturbations in these atmospheric parameters. The NASA/MSFC Natural Environments Branch recently released Earth-GRAM 2016 to the user community. A main feature in Earth-GRAM 2016 was converting the code from Fortran90 to C++. The Natural Environments Branch intends to make future improvements to Earth-GRAM as well as investigate the availability and improvements of model reanalysis for data input. The paper will describe an overview of Earth-GRAM, examine future updates, and identify areas of needed improvement.

Below are quoted from: https://software.nasa.gov/software/MFS-32780-2 

Earth GRAM 2016 is a computer code that can run on a variety of platforms including PCs and Linux stations. The model provides values for atmospheric parameters such as density, temperature, winds, and constituents for any month and at any altitude and location within the Earth's atmosphere. Earth-GRAM 2010 is available in FORTRAN.