Trying to make a modified map of South America with real-world height data

[Turambar]

I'm trying to create a map of a future, post-apocalyptic South America with elevated sea levels and some topographical changes, but I want to start with present-day South America height map as a baseline, and I'm having a lot of problems with that.

My tools are GIMP (2.8 is my preferred, but I have 2.10 and can use it), WILBUR, and Inkscape (which I use for icons and labels, not much use for this part of the process)

I've been trying to pull USGS data from Earth Explorer, and while I've tried multiple data sets, the only one that includes all of South America and isn't ludicrously over-defined (I'm wanting to make an image that's no more than 3300x5000 pixels) is GTOPO30. This data can come in two formats: DEM and GeoTiff.

DEM doesn't work with GIMP. I get some layers, but nothing I can use as a height map. I'm not sure what program I could use to play with DEM.

Geotiff is really finicky with GIMP; it doesn't work at all with GIMP 2.8, and even 2.10 sometimes refuses to open it, and cites errors when it does. That said, I did manage to open the four source map images, stitch them together into a coherent 9,600x12,000 pixel map, and shrink it down to something manageable. Problem is, the height map is almost a pancake. The difference between lowlands and mountaintops is barely noticeable.

South America Real Life.png

So I tried using the Color Curves tool to make the elevation differences more pronounced, which helped, but this yielded a tiered look to the terrain, and most of the continent appears totally flat and at the same elevation. This is a problem, especially when I want to play with sea level effects on the coasts; it means that either the coasts aren't going to be affected, or most of the continent will be underwater.

South America Attempt 1.png

So this is my problem: I need a way to get height map data for South America that I can open with GIMP and that uses the full spectrum of black to white to define heights.

I haven't found any good tutorials that seem to cover this. Any ideas?

A couple thing. First in Gimp, you can use levels to set the highest and lowest values as black and pure white respectively, which should help with contrast (make sure your DEM is as high resolution as possible for best effect).
Another thing you can do, is pull the DEM into a program like Gaea to add texture to it before working it in Gimp. For example I actually did a quick experiment with South America not long ago:
SA map.png

[Coriolis]

A couple thing. First in Gimp, you can use levels to set the highest and lowest values as black and pure white respectively, which should help with contrast (make sure your DEM is as high resolution as possible for best effect).
Another thing you can do, is pull the DEM into a program like Gaea to add texture to it before working it in Gimp. For example I actually did a quick experiment with South America not long ago:
SA map.png

Thanks for the tip with the levels setting. The result is much better.
South America Experiment 2.png
It's still not quite perfect; it's very steppy at lower elevations, but it's much more workable than what I had before. This might just do the job.