You can run a QTP script as a GUI user via LoadRunners controller, but it is and always will be a QTP script.
Unless you are using Citrix or Terminal Services you can only ever run one GUI user per Load Generator, so its usage is some what limited in terms of inducing 'load'.
I would primarily run a GUI user as part of a Load Test in order to establish the overhead of client side processing for a transaction, as versus calling the server API directly (as is the case with LR scripts).
On a more fundamental level: The objective of QTP and LR scripts is generally quite different. QTP is designed for functional testing (in other words, it is used when there is some expectation that the application might NOT work correctly). LoadRunner scripts, on the other hand, usually assume that the application DOES work (at least, from the functional point of view) so they contain little if any error handling for functional errors.
For that reason, I would imagine that very few users of both LoadRuner and QTP (or WinRunner) would consider that "porting" a script - unchanged - from one product to the other would be a good way to proceed.