The Gatekeeper is able to track Harry in some fashion.
"What are you doing here?"
"Looking for you," he said.
"You've been watching?"
He shook his head. "Call it listening. But I have had glimpses of you. And matters are worsening in Chicago."
The Gatekeeper may not be tracking Harry directly. We have so little information to work with. For example, if I were to ask forum members to give an explanation of what the Gatekeeper meant when he said, "Call it listening," I bet every response would be unique, and we wouldn't have a way to confirm any of them. (So please don't try unless you can quote something directly from one of the novels to back you up.) To get back to my main point, whatever the Gatekeeper may be perceiving, it's possible Rashid is looking for black magic, or to steal a Star Wars term; "a disturbance in the Force," or Mab might call it, "A disturbance in the balance of forces," and Harry is often at or near the center of those disturbances, so that could be how the Gatekeeper gets glimpses of, and eventually finds Harry.
Only in Turn Coat, after Harry called the White Council and told them he had Morgan, can we safely assume the Gatekeeper was directly looking for Harry. Even then, we don't really know if Rashid saw where Harry was, or if he saw, heard or felt something that told him to go to Demonreach without knowing; I mean having having 100% knowledge, that Harry was there. It could have been, the Gatekeeper's search only revealed to him that he had to get to the dock on Demonreach to learn the truth. He could have easily guessed that Harry would be there, but that's not the same thing as being certain that Harry would be there.
Unless the Gatekeeper gives us a more detailed explanation of how he gets these perceptions; which seems really unlikely, or Harry gets to use the Outer Gates material himself to do some kind of magic search; which could happen in a future story, we don't have a solid explanation of what the Gatekeeper does or what he's looking for. By the way, I'm fine if we never do. Not everything in the Dresden Files requires a detailed explanation.