From Original War Support Wiki
Tutorials -> GM Editor
Gm Editor allows you to edit and create GMX, GMS and GMZ files. Effectivly its a paint package designed for Original War only files. Currently it doesn't have an import image or a palette editor, but you can copy and paste from the clipboard. Also for palettes you can use OS Palette Editor and import them.
In other words, with this tool you can create buildings,units,etc.
You need to "paint" them in the frame.If it's ready move on to the next frame,... You create the movements on this way like crouching for example.