You should use a different view for sfml and tgui if you want to render them differently. Because you want the gui view to change with the resolution, you can't just pass true to the draw function (which is for changing sfml view while keeping the gui view).
So you will have to do something like this:
set sfml view
render with sfml
set view for tgui
render the gui
How to change the view is something that is explained in the sfml tutorials.
So you will have to do something like this:
set sfml view
render with sfml
set view for tgui
render the gui
How to change the view is something that is explained in the sfml tutorials.

