Gates reportedly prepared for his testimony with the help of Jake Greenberg, who previously was the Oversight committee's chief investigations counsel.
Houses across the city have been set on fire following a knife attack earlier this week.
The US president warns Iran "will have to pay the price" for taking too long to agree a deal, after Tehran vows retaliation to any attacks.
Inflation jumped to a three-year high in May as the Iran war continued to fuel higher energy prices.
A night of disorder in Belfast saw people forced to flee their homes, in protests that followed a knife attack in the north of the city.
"This is similar to a doctor that is licensed to practice family medicine but is doing brain surgery in their office," police said.