You would have to first be booked into a Magic Kingdom view room (Theme park view) Lagoon View is not the same as TP view (Some have been disappointed by this.)
The MK is a bit distant to the Grand Floridian, but you would be able to see parts of it. Unfortunately TP view does not always include a great view of the castle, all that's required to be called TP view is a view of the park...could easily be splash mountain instead of Cindy's castle.
Make sure to book TP if you really want that view, and put Castle in the requests, and then at check in verify what your view really is.