A news release from the Prime Minister's Office said the meetings will let the Microsoft founder and Harper "discuss opportunities for future collaboration, especially in the area of maternal, newborn and child health."
Canada has made it a goal to improve maternal, newborn and child health around the world, using its platform as host of the 2010 G8 and G20 meetings to push for increased funding. Since then, the government has pledged more than $6 billion to those targets.
This year marks the deadline for the UN's millennium development goals, which include improving health for mothers, newborns and children. The UN will set its next set of development goals in the fall.
The Prime Minister's Office said Gates and Harper are expected to discuss ways to keep maternal, newborn and child health issues global priorities.
Gates and Harper will participate in a roundtable with international aid organizations at 2:50 p.m. ET in Ottawa.
The meetings with Trudeau and Mulcair will also take place this afternoon.