GPP president Gordhan Zadaphia announced this decision at the BJP headquarters here.
The GPP took the call, much on the predictable lines, even as former chief minister and one of its founder members, Suresh Mehta, had opposed the move.
Terming the merger illegal and a unilateral call on the part of Zadaphia, Mehta said he would continue his fight against Modi.
GPP was formed ahead of Assembly polls by BJP dissidents and former CMs Keshubhai Patel and Suresh Mehta, along with Gordhan Zadaphia, then a junior home minister in the Modi Cabinet.
"The son of Gujarat is going to become the Prime Minister of India. We will support the cause. We had some difference of opinions when I left the party. But our differences should not come in the way of the bigger interest of the nation. We have worked as the swayamsevaks and will remain swayamsevaks for the national interest," Zadaphia said.
Zadaphia insisted that the decision to rejoin BJP was taken through consensus.
"BJP is our alma mater and this decision of merger has been taken unanimously in presence of all district presidents and secretaries of GPP. The decision is taken considering the future of GPP workers, and to serve the nation. We all wish that Narendrabhai becomes PM," the GPP president said.
GPP had suffered a body blow when Keshubhai's son Bharat switched over to BJP and later the veteran leader himself stepped down as party president and MLA to announce his retirement from politics.
"There will be an official function when top state leaders including Narendra Modi will welcome GPP members at a function tomorrow," BJP spokesperson Harshad Patel said.
Zadaphia had first formed MahaGujarat Janata Party (MJP) when he walked out of BJP on the eve of 2007 Assembly polls owing to his differences with Narendra Modi. Later, he merged his party with GPP in 2012. Mehta said that he did not recognize the merger with BJP.
"GPP can never merge like this. Nobody can take the entire party to another one. The party has been registered legally and Zadaphia has no right to merge it. I oppose this merger. Though he is a president, if he wants to go he can go with his men, but party will never merge," the former chief minister said.
Mehta said almost two third of GPP cadres will not follow Zadaphia in the merger.
"Merging with a party like BJP can never be possible. Let Zadaphia go, I will remain here with my party," the former BJP leader who became CM for the first time in 1995 said.
Meanwhile, GPP MLA and former BJP member Nalin Kotadiya also joined BJP on Monday along with Zadaphia.