Hi,
I work as a GNSS engineer and I have experience in processing data coming from INS. In addition, I have a BSc in Geodesy and a MSc in Geomatics.
I intended to use some of my matlab scripts to complete the task, based on what you say, I would need to know the setup of your INS in order to compute the correct rotation matrix. Also, what environment were data collected in order to make some assumptions on the errors would be needed. It will be helpful to know some information about the INS system too. In any case, even if these are missing I can compute the Allan Deviation graph that would allow me to completely characterize all stochastic errors of the accelerometer and gyroscope. Additionally, I would like to know a bit more about the GPS data and in which reference system you need the coordinates ( XYZ in WGS84 or latitude, longitude, height in WGS84).
I am available on private if you are interested in discussing more details about your project.
Thank you for time and consideration.
Regards,
Florin