I don't believe the map editor can generate random maps, no. It's a function I thought about adding but I didn't get around to it.
The gapfill string lists a number of map feature types (the numbers correspond to the types listed in randmaps.txt) which will be inserted into empty spaces on the map. To illustrate how this works, open up randmaps.txt in your editor and look at Map Type 1, the "Default Wilderness".
The rendering command for default wilderness is "Fill 1 0", which simply fills the entire map with "Open Ground" terrain. The next line, the GapFill line, is where the real action takes place. It lists five map feature types: "-4 -5 -9 -4 -5", which correspond to Forest, Hills, Lake, Forest, and Hills.
So when a default wilderness map is created, first the entire map is filled with open ground. Next, the map generator starts placing randomly generated map features in any empty spaces that it finds. Because forest and hills are listed twice in the gapfill string they are each more likely to be generated than the lake is.
If a scene has any predefined map features, these get placed first before the gapfilling features are generated. Map features can do all of this recursively, down to a size of about 5x5 tiles. See how Hogye is constructed for an example.
A map generated with the map editor can't be only partially defined, since a pregenerated map loaded from disk doesn't go through the random map generation process. In GH2 it is possible to define a 5x5 minimap and insert that into a scene, and it's also possible to assign map feature anchor spots in a pregenerated map, but neither of these things work in GH1. Sorry.
