if you need more help, I can tell you.
USBLoaderGX will expect files located here :
The gct : SD:/codes/G8ME01.gct
The game : /games/Paper mario 2 [G8ME01]/game.iso
- game can be on SD or USB
- the "paper mario 2" part can be anything else if you want
- it's better to use subfolder and have the TitleID in the folder.
Launch USBLoaderGX
go to "top menu" 4th icon, select gamecube option
It should list your game now
if not, let me know.
click on the game, and select "option" then "Ocarina"
it will tell you "there's no cheat list, do you want to download it?" answer yes.
if "geckocodes" have that game, it will download all the cheatcode you can use.
if "geckocodes" website does not have the game, you probably have the .txt in your "codes" folder that you made a screenshot in your first post. copy that .txt in your txtcodes/ folder
if you don't have any "txt" file, and geckocodes website does not have this game, then you can't see what is inside the gct, sorry.
if everything is fine, the cheatcode located in the gct will be marked as "enabled".
from that menu, you can enable/disable cheatcode, and click generate gct to replace it with a new gct file if you want.