Create a .cht File
Open Notepad and enter your cheat codes. Label each individual code with a header that starts with the exact version of the cheat hardware, followed by a custom name you can enter for the cheat. For example, if you are using Gameshark Version 3 and your cheat is named "InfiniteLives," you use the header "gameshark_v3 InfiniteLives" and paste the codes below it. When you save your file, enter the extension ".cht" to the end of the file name and match the rest exactly to your ROM file. For example, if your ROM's name is "Final Fantasy V Advance.gba," your cheat file name will be "Final Fantasy V Advance.cht." You want to create one cheat file per ROM.
GameShark Cheats
GameShark cheats are typically strings of 12-character hexadecimal numbers. Codes may be as short as one number but typically range from two to four numbers or higher. An example GameShark code for the game "Final Fantasy V Advance" is the "All Jobs Mastered" code, which is entered as "CB354DE36B76 8B313DB5DA3F" in gpSP or any other GameShark-compatible device.
Action Replay Cheats
Action Replay cheats are represented by eight-character hexadecimal numbers arranged in a string of one or more individual numbers. Though many Action Replay cheats mirror GameShark cheats in functionality, the codes themselves are slightly different. For example, the "All Jobs Mastered" code for "Final Fantasy V Advance" is "21afda85 1637547f" for Action Replay.
Enabling Cheats in gpSP
Move your ".cht" file to the same directory as the ROM file you are planning on playing. Open gpSP and start playing the game. Go to the "Cheats/Misc" menu and enable or disable each cheat.