Speaking to journalists on his campaign plane after the incident in London, Ontario, Mr Trudeau said he may have been hit on the shoulder.
According to a reporter with Canada's CTV National News, two people travelling on a media bus were also hit by the gravel, although they were not injured.
On Tuesday, the Liberal leader called the actions of the protestors "absolutely unacceptable".
"Nobody should be doing their jobs under the threats of violence or acts that put them in danger," he said during a Montreal campaign stop. "But it's not just political rallies where this is happening."
Provincial politicians, public health officials, restaurant owners and healthcare workers have faced harassment in incidents across the country in recent weeks.
Erin O'Toole, the leader of the opposition Conservative Party, described the incident as "disgusting".
"Political violence is never justified and our media must be free from intimidation, harassment, and violence," he tweeted.
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