Obligatory not a lawyer/accountant:
What Alby said is important to keep in mind about Argentina broadly (the law vs. real life), and, the later part even more so: I've yet to hear of/meet a single retiree here whose had to pay ganancias or any sort of tax. That being said, they've all been average Joes with Social Security checks, nothing to brag about.
Now, if you want to own real estate, spend thousands of dollars a month, have a new car (or even used in this economy) etc. it will likely be worth meeting with an accountant to figure out your options.
The only tax I've heard retirees here ever having to pay is IIBB (ingresos brutos) and that's because they have no declared income and make purchases with debit or credit cards/receive money from WU so the provinces apply the 5% auto deduction for any deposits/tranferencias received.