The company sold about 28.2 million shares in Africa’s pharmaceutical manufacturer for a price of ZAR372 per share.
After completion of the sale, GSK will remain a 6.2% shareholder in Aspen and has undertaken not to sell any more shares in the South African firm for 180 days.
GSK chief financial officer Simon Dingemans said: "GSK has a long and successful commercial partnership with Aspen and our investment in the company has grown in value significantly over time.
"As we continue to reshape the Group around our core franchises and drive the benefits from the Novartis transaction, optimizing our financial flexibility to invest behind these priorities is key.
"As a result we have decided now is the right time to realise further value from this successful relationship. We continue to believe in the strategy of Aspen and we remain committed to working together in the future."
According to the company, proceeds from the transaction will be used for general corporate purposes.
Citigroup Global Markets and UBS have acted as the Joint Bookrunners in relation to the transaction.