Firstly don't use anything below gfa v3.6. The original gfa editor should work in any mode that is 16 colors or less. Stick to standard resolutions though or the compatibility modes on the Falcon. GFA dropped support about the time the Falcon came along, so it don't like 256 colors and up. If it still don't work, then something is weird on your machine or conflicting with it.
There are also patches, like 'Bastard' and 'Ala Card' to solve the video issues. With 'Bastard' I can run the old editor at a glorious 256 colors at 1280x1024, even under MiNT. However I have switched to using my own GFA editor. The editor I made will work in any mode, on any video card, etc.
I have tried Omikron a few times myself, but it's a bit unstable (the editor) on my Hades. Lucky if it runs for more than 20 minutes. Perhaps it don't work well with MiNT? For me that's a show stopper, such things end up visting the trash icon.