Delta is not a linear function, meaning it will not change proportionately with the stock price. Gamma quantifies how Delta will change per the change in the stock price. The chart below shows the non-linear “S-like” shape of Delta and the Gamma curve that warps it.
A Gamma squeeze relies on the hedging actions of options dealers.
The banks and brokers who are the largest sellers of options must hedge their trades. Most dynamically hedge, meaning they frequently adjust the hedge amount according to the Delta of the option. If the Delta is .35, then they buy 35 shares for every option contract they are short. If the Delta then rises to .40, they buy five more shares. Conversely, they sell when the Delta falls.