.Png To .Raw Conversion

I’m trying to convert my .png file to a .raw file. for terrain. How should I go about doing this? And before you ask, I don’t have Photoshop, I have GIMP. Please help.

@ Coleclaw199 if you don’t mind spending $10 then you could use Terrain Importer on the asset store
It can import png, ter, tif, tigg, raw 8bit, raw 16 bit, r16 and r32. It does not convert the file but imports it straight into the terrain, works with new or existing terrains plus works with single or multiple terrains.
I have found it to be a most useful tool.