First, i don't think that this thread goes here, its made for toribash-related support.
Second, If you know, .XCF files aren't images, its a gimp file to keep/save layers, paths etc...
So if you want to save it as .tga files, Go to Export, Type the name and at last put ".tga".
Or if it lets you to choose file type, select TGA and just write the name without ".tga".
Also, the textures aren't lost yet if you still have the .xcf file, open it and you will get the texture you made.