How can i reduce these GDI and User objects in .NET MDI form

Well, if you're not doing it already, one thing you can do is to call Dispose() on these objects when you no longer need them (to release unmanaged resources) and set any references to them to null so that the GC can (in due course) clean up the managed memory they use.

Rather than creating new objects, I'd also consider reusing existing objects where feasible by assigning references to them to fields rather than to local variables so that they are available to all methods in your form.