IDRx, a privately held biotechnology firm, is developing a therapy for the treatment of gastrointestinal stromal tumours.
The deal, which comprises an upfront payment of $1 billion, will add to GSK's growing portfolio in gastrointestinal cancers.
The British drugmaker, which has been grappling with a decline in its vaccines businesses, has stepped up its efforts in cancer treatment in recent years.
"This acquisition is consistent with our approach of acquiring assets that address validated targets and where there is clear unmet medical need, despite existing approved products," GSK's Chief Commercial Officer, Luke Miels, said in a statement.