There are a few ways that you can hack a GTA keypad. The first way is to use a keypad overlay. This is a piece of paper that you can place over the keypad that will show you the correct key presses.
The second way is to use a keylogger. This is a piece of software that will track the keystrokes that you make on the keypad.
The third way is to use a physical device that can be attached to the keypad. This device will mimic the keystrokes that you make on the keypad.
All of these methods can be used to hack a GTA keypad.