Gilroy has featured in both of Ulster's Heineken Cup victories this season as they sit at the top of Pool 4 but face a crucial December double-header with Northampton Saints in Rounds 3 and 4 of the competition.
The fleet-footed 21-year-old, who has signed a new three-year contract to remain at Ulster until the summer of 2016, announced himself on the European stage with a spectacular try in Ulster's victory over Munster in last season's Heineken Cup quarter-finals.
The touchdown, Gilroy's third European effort, was voted Try of the Season at the 2012 IRUPA Player Awards and he has continued his rapid progress.
He bagged two tries for an Ireland XV against the Barbarians in May and grabbed that hat-trick last Saturday's non-cap international against Fiji at Thomond Park.
Meanwhile, in Saturday's Test against Los Pumas, Gordon D'Arcy and Keith Earls will continue their centre partnership, while Jonny Sexton has recovered from the tight groin he suffered in the warm-up for the Fiji game and starts once more in the No 10 jersey.
Sexton, who is now fifth on the list of Ireland's record points scorers, is just two points off notching up 250 Test points for his country.
The forwards selection is the same that lined out for the opening game of the GUINNESS Series against South Africa, with Richardt Strauss winning his second full cap in the front row.
The team will be captained by No 8 Jamie Heaslip for the third consecutive game in this month's Series.