Market maker
A firm that quotes bid and ask prices all day and takes the other side of customer orders. In options writing it is often called the dealer.
A market maker posts prices at which it will buy (the bid) and sell (the ask), and earns the spread between them when customers trade. Because it fills orders on both sides, it ends up holding whatever customers do not want, such as calls that investors sold or puts that investors bought.
Market makers usually aim to hold little directional risk. Instead of betting on where the stock goes, they offset the delta of their option book by trading the underlying stock or futures, which is called delta hedging. GEX models assume this hedging happens and estimate how large it may be.
Illustrative example (hypothetical numbers, not a real trade): a customer buys 10 put contracts and a market maker sells them. Each put has a delta of -0.30, so the market maker is now long 10 × 0.30 × 100 = 300 shares of delta exposure. It sells 300 shares of the stock to stay roughly neutral.