I've been trying to figure out for a while now, to no avail, how to merge two types of terrain. Can't find anything related myself so I'm asking for help here.

By merging two terrain types I mean making a smooth transition from one terrain type to the other, say from snow to grassland and then desert or whatever.

GIMP is my program of preference.

I get a feeling it's something quite simple that's been staring at me this whole time. Point me in the right direction if there's a tutorial of a sorts but spare me the torment if the answer is too complex and beyond the skills of a simpleton like myself.