Oct 2 (Reuters) - OpenAI, the company behind ChatGPT, has reached a valuation of $500 billion, following a deal in which current and former employees sold roughly $6.6 billion worth of shares, a source familiar with the matter told Reuters on Thursday.This represents a bump-up from its current valuation of $300 billion, underscoring OpenAI's rapid gains in both users and revenue. Reuters reported details of the stock sale earlier in August.Learn about the latest breakthroughs in AI and tech with...