@myselfhimself, in current developpement branch of G’MIC 2.9.5, command elevation3d has been re-implemented as a custom command (rather than a native one as before).
It has a new option (second argument) to allow the generated 3D elevation to have a solid base , which is something you were looking for.
In your case, I guess that your elevation factor should be negative. Example:
$ gmic 0 text Chocolate,0,0,48,1,1 b 1 n 0,255 elevation3d -0.02,10 o output.obj
generates the following 3D mesh:
If you want to test it, let me know if you get into troubles (or not 
).