The Alpha Research poll showed GERB winning 26.4% of the votes, while reformist We Continue the Change (PP) was seen coming second with 14.9% of the votes.
The ultra-nationalist Revival party was set to come third with 12.9% of the vote.
Sunday's vote, the seventh in four years, was triggered by the failure of Bulgaria's political parties to agree on forming a coalition government after an inconclusive election on June 9.
Bulgaria has been run by short-lived governments since 2020, when anti-graft protests helped to end a coalition led by the GERB party.
GERB would get 74 seats in the 240-seat parliament, while PP would get 42 seats, and Revival 36 seats respectively, according to the Alpha Research poll.