That's what I tried too, and encountered the same problem. I think the part of the process that's missing is either conversion of all colors to the SPMBT palette (not sure how to do that using GIMP), or that the 36-image converter app doesn't properly convert the file.
I don't want to spoil your day, but are you sure that GIMP will save to .lbm format?
I just tried and couldn't find it in the format list.
Or maybe you have something separate for the conversion? That would be interesting.
There's a freeware app called the "36-Image Converter," which Urban has mentioned, and which I have also attempted to use. The GIMP will not save to lbm, but the 36IC says it will convert to that format.
I tried with GIMP but it didn't work - however Paint Shop Pro worked just fine.
Urban, if you're unable to get your hands on Paint Shop Pro send the pictures to me and I could edit/save the lbm's for you. I have some experince with that stuff.