BlackRock Inc, the world's largest asset manager, could have made gains of about $2.4 billion on its investment in GameStop Corp as retail investors pour money into the video-game retailer's shares. Assuming no change in BlackRock's position, the value of its stake would be worth $2.6 billion now, compared with $173.6 million as of December. GameStop's stock has surged nearly 700% in the past two weeks in a rally fueled by interest from amateur investors on Reddit, forcing hedge funds to retreat from heavy losses and sparking calls for scrutiny of the social media-driven trading frenzy.