The Brit clinched the top honour in the Grade II individual dressage after racking up a total of 82.800 for her dressage routine.
"I am completely over the moon," said Baker. "It was a personal best, a Paralympic record and a gold medal. That's pretty special.
"I just wanted to come and do my best, and I did. The music suits my horse so well. It is perfect for him."
Baker claimed her first gold of the tournament in London after winning Grade II of the individual championship on Saturday.