Custom figures are very easy in GMF if you want to use polygon-based representations. I wanted a custom figure as in the diagram on the right.
The solution was to replace the Rectangle with a new Scalable Polygon and keep the previous contents. You also need to add Template Points for each point in the polygon, noting that the first and last points will join up, i.e.:
The change in the .gmfgraph looks like this.
If I used just a normal Polygon, nothing would render.
For more information check out the GMF Tutorial Part 3 (except this doesn't mention using Scalable Polygons).