Thanks for your posting and using Aspose.Cells.
If GoalSeek is a normal Excel function and not a Macro or VBA function, then CalculateFormual() will calculate it. However, if it is Macro or VBA function, then you will have to implement ICustomFunction interface yourself which will contain the logic of your Macro/VBA function.
Please see this article that explains how to implement ICustomFunction interface to calculate custom functions with the CalculateFormula() method.
( http://www.aspose.com/docs/display/cellsnet/Using+ICustomFunction+feature )