WiiFlow can handle making GCT files. It may depend on version (I'm probably using an older version) but I have cheat codes listed on the second page of an individual game's settings.
Again, may this may depend on version, but TXT cheat codes are stored in
/WiiFlow/codes
You can read about the geckocodes.org shutdown
here, and you should still be able to get codes from its mirrors. I think the easiest way to do it is to
find a game, click the "↓txt" button, and right-click and save the page (not sure if this works at the moment) or copy and paste its contents into a text file.
The file name should be the same as the game ID, as shown in the URL
So this would be named RMGE01.txt
Once saved, place it in /WiiFlow/codes and it
should be recognized when viewing the cheat codes menu.
If you are using WiiFlow Lite, you won't have to deal with this anymore once the next release happens as it will use the RC24 mirror for downloading cheats. Then, you should be able to download them easily from the cheat menu if you don't already have them.