What GRID does Taverna run on?

Taverna doesn't necessarily run "on a grid". It is a desktop application that can be used to access resources, either on a grid, or anywhere else. Taverna can, however, be installed as an application on a grid node to execute workflows using the command line tool.

Since Taverna workflows are created by combining a variety of web services "out there", you could say that a workflow uses or even forms a grid of services and data.