Wikipedia says:
"A gömböc or gomboc (Hungarian: [ˈɡømbøts]) is a convex three-dimensional homogeneous body which, when resting on a flat surface, has just one stable and one unstable point of equilibrium. Its existence was conjectured by Russian mathematician Vladimir Arnold in 1995 and proven in 2006 by Hungarian scientists Gábor Domokos and Péter Várkonyi."

This is a rescaled Gömböc from seanmichaelragan. It has been separated into 2 parts for easier printing and possibility to add material Inside. Dimensions are 90 mm x 100 mm for each shell.
Print two shells and bond them together to get a Gömböc! The idea is to fill the shells with Synthetic plaster. It is not very expensive, and liquid enough to fill the half shells. It is very fast to dry, so be quick! It is not easy to sand but liquid enought to fill accuratly all the inside of the shells and very heavy. Bond the shells together at the end. On the picture, I also sanded the outer perimeters a little bit and added a product to smooth the surface in order to get a good looking Gömböc.

Printing parameters:

Shell thickness: 0.4 mm
layer thickness: 0.2mm
fill density: 10%
Bottom thickness: 0 mm
Top thickness: 0.4mm

Speed:
Outer perimeter: 30mm/s
Filling: 50mm/s

Printing time: about 3h20 for each shell

Caution:
Do NOT use epoxy, as it gets very hot when polymerizing (it melted the PLA), and do NOT use polyester because of solvent (which ALSO melt the PLA).

Disclaimer:
This is not a real Gömböc, mathematically speaking: As the real one is fully homogenous and has the very correct shape, its behaviour is just as intended. By printing this Gömböc, the material is not homogenous anymore because of the filling percentage (10%). By filling the half shells with heavy material, you get a more accurate Gömböc, but is still not homogenous.
you will need to add some weight at the bottom of the Gömböc in order to make it working as expected.
Don't worry if the shape or the CoG is not exactly where they have to be: Maybe your Gömböc won't be a perfect mono-monostable object as it can meet other unstable equilibrium points due to the unperfect shape or balancing. But who cares? Finding these unstable points is almost impossible, and your Gömböc will just behave as expected in the real life.
For purist people who really wants to have a perfect Gömböc (mathematically speaking), visit the Gömböc website: http://gomboc-shop.com/