A GBA Doom editor made by kgsws, so it isn't mine, but I'm reuploading it here.
- Full texture/sprite replacement support.
- Partial graphics replacement support (images stored in WAD).
- Sound replacement support.
- DeHackEd-like editing support for states and actors (with code-pointers).
- Internal storage (ROM size) is limited, so it's recommended for you to remove unused graphics (yes there is plenty of unused graphics inside) before you start editing.
For map editing, you'll need a nodebuilder that generates a GBA-Doom-Compatable LEAFS
lump. A nodebuilder called LBSP.exe comes with svKaiser's GBA Doom tools which will do this job.