If you go to the class reference help on GridStyleInfo.CellType, it lists 20 celltypes there.
Another way to see a list is to open up a form with a grid on it in the designer. Then click the grid and open up the TableStyle property in the propertygrid. Then open up the CellType. This should show a dropdown with a list.
You can add your own celltypes (and strings) using grid.Model.CellModels.Add so you can very well have many more valid strings than the 20 listed in the help.