In that kind of context, I would suggest to build your home made mvp framework. It would be easier and cleaner than using a subset of the capabilities of GWTP.
That being said, it's not imposible, but you will have to use a complete hierarchy of presenter / presenter widgets. But that won't mean that the lifecycle will work... The lifecycle of GWTP is strongly tied to internal Navigation Events and if your application doesn't use this feature of GWTP, you're likely to lose all its advantages.
Here's how you can embed a GWTP application in your own already existing application: https://github.com/ArcBees/GWTP/wiki/Website-Embedding
We've already worked on migrating an existing application to GWTP by embedding the existing application IN GWTP while gradually migrating the existing code base, but not the other way around.