I am selling the property that I bought in 1998. At the time the price was expressed in francs. Is the notary obliged to use the INSEE converter to determine the amount of the transaction that shall be carried out in euros ...

... and calculate the capital gain for which I am liable?

No. The INSEE converter is a tool that makes it possible to determine the equivalence between two currencies. It takes into account the currency depreciation due to inflation. Nevertheless, to the extent that this tool works on the basis of statistical data, INSEE specifies that this data "cannot be a legal reference". (https://www.insee.fr/fr/information/2417794).