GRSTGammaInc stands for Amparo Gil, Diego Ruiz-Antolín, Javier Segura, Nico M. Temme's Incomplete Gamma. These are the original authors of an incomplete gamma function for negative numbers.
It is based on this paper, plus the provided Fortran implementation.
This repo contains the Fortran bindings, and (eventually) some test cases and (eventually) a C translation of it.
The license that this code is offered under is unknown. Per ACM policy, "For Algorithms published in 2013 and after, the author holds the copyright. The terms of use for these Algorithms follow from the license the author(s) of the Algorithm has chosen."
The original Fortran code is rumored to be licensed for inclusion in the SciPy project.
If this is too much legal risk for you, then don't use this repo.