ScottishPower Renewable's parent, Spanish utility Iberdrola, will receive £1.63 billion (€1.75 billion) for the sale and retain a 60% stake in the contracts for difference-backed project.
The transaction therefore values the project at £4.1 billion (€4.4 billion), Iberdrola said.
GIG — part of Australian investor Macquarie — said following this transaction, it will have supported over 5.7GW of offshore wind in the UK.
The site is currently under construction 42km off England's east coast, with the first turbine installed in June 2019. When completed in 2020, it will comprise 102 Siemens Gamesa 7MW turbines, with 154-metre rotors.
The deal is Macquarie's second offshore wind investment this month, after acquiring the Bard Offshore 1 project in the German North Sea in early August.