There are plenty of choices that we could use temporarily, the only thing we would have to do is just tile them again in whatever solution we come up with.
The main advantage of using an external editor would be in the object handling as if you were to map it in game maker, you wouldn't be able to place objects until after the files are merged into the main build, otherwise you would end up with duplicates.
Not really having looked at GMarc or Infini's editor it would be hard to say which would be the best option. I'm already biased towards mine, mainly due to features that I have in planning for it.