PNG to RAW without photoshop?

So yeah… I can’t afford photoshop and I have Paint.NET and Gimp. Here is the heightmap I’m trying to use

I just googled “png to raw” and I found this site No promises if it works, but I assume it does.

I believe the current version of GIMP only supports 8-bit raw, and Paint.NET doesn’t at all.
I create heightmaps using GDAL - free and powerful command line utilities for working with all sorts of raster and vector data. The syntax can be a bit hard to learn, but I wrote up an example of converting a TIFF file here (PNG input will work just as well) - Importing DEM Terrain Heightmaps for Unity using GDAL | Alastair Aitchison