Halep defeated No.10 seed Gauff, who had beaten Naomi Osaka, Elena Rybakina and Aryna Sabalenka this week, in one hour and 47 minutes.
The No.15 seed from Romania was broken five times and sent down eight double faults, but found the edge with speed, finesse and power.
The win means Halep has won the third most matches (36-10) on the WTA Tour this season, behind only No.1-ranked Iga Swiatek (49) and Wimbledon finalist Ons Jabeir (37).
Halep's improved form comes after she teamed up with Serena Williams's ex-coach Patrick Mouratoglou in April after a difficult 2021 season.
"Before I met him I was super down with my motivation," Halep said. "I couldn't really work, keep working. I was almost done with tennis. He brought this fire back and the motivation. He trusted that I still can play good tennis and he transferred this to me."
The 2018 French Open and 2019 Wimbledon champion added: "Physically I'm good. Mentally I'm good, so I think it's all positive."
Halep, who won the Canadian Open in 2018, has reached the semi-finals for the fifth time in her career where she will take on No.7 seed Jessica Pegula.
Pegula got past Yulia Putintseva 6-3 6-3 in one hour and 21 minutes. Qualifying for the last four at the Canadian Open for the second straight year.
Brazilian Beatriz Haddad Maia maintained her Canadian Open run, having ousted Swiatek in the third round and local favourite Leylah Fernandez in the second, with a 2-6 6-3 6-3 victory over Tokyo 2020 Olympic Games gold medalist Belinda Bencic in two hours and 11 minutes.
Haddad Maia became the first Brazilian to reach the semi-finals of a WTA 1000 tournament. She will face Karolina Pliskova in the semi-finals after the Czech No.14 seed beat Qinwen Zheng 4-6 6-4 6-4.