Polestar Colchester is producing the weekly medical title.
"It's fantastic to have Haymarket back," said Polestar chief executive Barry Hibbert. "It's a shot in the arm for Colchester and the group."
Defunct web printer TPL previously produced GP, although both firms said they had been in negotiation regarding moving GP to Colchester prior to TPL's demise.
Hibbert and Haymarket production director Chris King signed the two-year deal yesterday following a three-month trial (Wednesday, 9 June).
Polestar has also won a further job from Haymarket a customer magazine produced for Nike.
"Polestar has taken two difficult jobs," said King. "GP is time critical, it has to get to market first in its sector or it goes straight in the bin, the priority for GP was on-time delivery. For Nike it was quality."
Haymarket also publishes PrintWeek.