If I change the parameter “factor” in the 3d elevation filter, I can’t see any differences between 100 and -100. For me this looks like a but.
The difference is here anyway :
$ gmic sp lena r2dx 100 b 2 +elevation3d[0] 0.2 +elevation3d[0] -0.2 rm[0] c3d +3d. 120,0,0 +3d
On the left: a positive elevation.
On the right : a negative elevation.
Positive elevation means your 3d points are further (higher z-coordinates) when they have a high elevation.
Negative elevation means your 3d points are closer (lower z-coordinates) when they have a high elevation.
OK, here are more information. 
I have G’MIC version 2.1.9 and Gimp 2.8.
Via command line it work her too.
But I used the Qt GIMP plugin. Here is the result from +/-100:
(Btw. it would be great if it would be possible to rotate the image interactive in the plugin preview like in the gmic command.)
Ah right, I wasn’t thinking about the plug-in filter, sorry 
This looks like a bug actually, I’ll try to fix that ASAP.
Fixed! Thanks for reporting!
Thanks for fixing.