So some people have reported the established codes not working in Dolphin, myself included. The thread about Dolphin needing offsets is old, and I think Dolphin has now changed the way it uses Gecko to now not need it. Anyhow, I made these, and they work for me:
https://forums.dolphin-emu.org/Thread-wii-fire-emblem-radiant-dawn--26089?pid=319200#pid319200
https://forums.dolphin-emu.org/Thread-wii-fire-emblem-radiant-dawn--26089?pid=319317#pid319317
http://serenesforest.net/forums/index.php?showtopic=47361
These are all Action Replay, not Gecko, I don't know what the difference is. You may find looking through the memory useful, using "Dolphin.exe /d" at the command prompt - there is also a Cheat Search function (Tools -> Cheats Manager -> Cheat Search), allowing you to compare memory before and after doing something, so try scanning the memory, doing something (spending gold in the shop for example) and scan again looking for things that are Not Equal, and see which memory addresses changed in that time - try it multiple times to get it narrowed down. Then you can use AR codes to set the memory to the value you want (e.g. amount of gold you have). I haven't done any changing of character classes yet. The problem you are going to run into is that a lot of characters have slightly different positions in the memory depending on whether you missed recruiting any characters. The first like 7 characters are pretty set but getting Aran or not getting him seems to be the first possible difference - it may also change depending on reinforcements and how many enemies you kill before recruiting different characters? I don't really know, I haven't experimented much. Clearly characters are placed in slots, not in a specific place.
Edit: Oh man, wall of text, missed that last paragraph, you know how the memory viewer works xP
Edit2: Actually, thinking about it there are indeed the "default" classes there in the code - I found them by looking for the skills they start with, sometimes Critical +5 or whatever - and adding skills to them seems to give those skills to any enemy of that class and any new player character of that class too (and anyone who promotes to that class).
https://forums.dolphin-emu.org/Thread-wii-fire-emblem-radiant-dawn--26089?pid=319200#pid319200
https://forums.dolphin-emu.org/Thread-wii-fire-emblem-radiant-dawn--26089?pid=319317#pid319317
http://serenesforest.net/forums/index.php?showtopic=47361
These are all Action Replay, not Gecko, I don't know what the difference is. You may find looking through the memory useful, using "Dolphin.exe /d" at the command prompt - there is also a Cheat Search function (Tools -> Cheats Manager -> Cheat Search), allowing you to compare memory before and after doing something, so try scanning the memory, doing something (spending gold in the shop for example) and scan again looking for things that are Not Equal, and see which memory addresses changed in that time - try it multiple times to get it narrowed down. Then you can use AR codes to set the memory to the value you want (e.g. amount of gold you have). I haven't done any changing of character classes yet. The problem you are going to run into is that a lot of characters have slightly different positions in the memory depending on whether you missed recruiting any characters. The first like 7 characters are pretty set but getting Aran or not getting him seems to be the first possible difference - it may also change depending on reinforcements and how many enemies you kill before recruiting different characters? I don't really know, I haven't experimented much. Clearly characters are placed in slots, not in a specific place.
Edit: Oh man, wall of text, missed that last paragraph, you know how the memory viewer works xP
Edit2: Actually, thinking about it there are indeed the "default" classes there in the code - I found them by looking for the skills they start with, sometimes Critical +5 or whatever - and adding skills to them seems to give those skills to any enemy of that class and any new player character of that class too (and anyone who promotes to that class).