Holden Gemler was a cybernetics specialist who lived in Van Nuys. He was hired to build a ring that would act like a tracking device.
Gemler was seen in his house with Sloane demonstrating how a new device he built for him worked. Gemler constructed a ring that would be the same as the frequency in his neck, but configured it so that it would appear that Sloane was still in league with The Alliance. In reality, Sloane was able to travel wherever he wanted to without having any worry that The Alliance was monitoring him. After demonstrating the ring, Sloane gave Gemler $10,000,000, and then killed him.
After he was killed, his body was discovered by Jack Bristow. He found out his left eye was blinking and was a fake. It turns out that Gemler was blind in his left eye so he constructed a device that would record what it saw, and after he died, the CIA found out that Sloane made a phone call regarding a place called Shipman.