Entering raw versus calculated data

Modelers often face two choices with respect to data.

The latter choice is often motivated by the availability of the raw data in a spreadsheet where those data could be manipulated before being put into GAMS.

My recommendation (as one who often updates models and needs to figure out the embodied assumptions) is put data in as close to the form it is collected into GAMS and then manipulate that data in the GAMS code.

Why?  Over time spreadsheets and other data manipulation programs change, or get lost.  Also often internal documentation in such programs is weak.  Putting it in GAMS keeps it all together.