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.
The Microsoft co-founder has repeatedly denied any wrongdoing or knowledge of his former associate's illegal activity.
"This is similar to a doctor that is licensed to practice family medicine but is doing brain surgery in their office," police said.
Cars and a bus were set on fire and families were driven from their homes as the stabbing attack caused tensions to spill onto the streets.