Dr. Gnivik is an antagonist in the non-canonical novel, Leo Rising. He had his minions kidnap the Charmed Ones and tested their limits in hopes of learning how magic worked in witches. He was viewed by others as a mad scientist.
Gnivik was a mad scientist who believes he knows everything, and will study what he does not in extreme ways.
After Leo Wyatt became mortal, the scientist allied with Zankou and sent his minions to attack the Charmed Ones to test their strengths, then had them kidnapped and taken to his lair for further experiments. He injected power depressants into their systems which turned off their powers.
When the Halliwell men (Leo with future Wyatt and Chris) figured out it was Dr. Gnivik who has the sisters, they deviced a plan to draw him out by faking a lecture at Magic School. Chris, pretending to be the student, gave the speech, but Gnivik interrupted immediately, thinking he was talking nonsense. The three men then followed him to his lair and broke in. He then strapped Phoebe to a table, Piper to a chair and trapped Paige onto a treadmill surrounded in a deadly force field. The sisters broke free from their binds, fought the doctor, and trapped him in the treadmill that ultimately became his doom.
Dr. Gnivik has appeared in a total of 1 non-canonical novel throughout the run of the franchise.