Am 25.10.2018 um 10:09 schrieb
Nobody can prevent one to call
inflation I in some .xlsx file.

That is true; however, if gretl finds illegal names in imported series (e.g. blanks, foreign chars) it changes them already. So in principle it would be easy to also make it illegal to have series named like built-in functions.
I'm not saying that this would be the optimal solution, but it's an option.