GSIP is currently seeking banking and legal advisers for the deal which could be made through a tender process or an initial public offering (IPO), the sources also said.
"The process is expected to be launched after the summer, after Spain's general election," a source directly involved in the process said.
Goldman Sachs, which bought Redexis in 2010, declined to comment.
The sources did not give any price estimate but analysts say the sale could fetch 1.75 billion euros (1.35 billion pounds) including debt.
Redexis transports and distributes gas to retail and wholesale clients in 26 Spanish provinces through a network of around 8,500 km of pipelines and 600,000 supply points.
It had a core profit or earnings before interest, taxes, depreciation and amortisation (EBITDA) of 136 million euros in 2015 while net debt stood at 840 million euros.
(Reporting by Andres Gonzalez and Jose Elias Rodriguez, writing by Jesus Aguado; Editing by Julien Toyer)
