I have never used gumpaste... granted I'm new and haven't made a whole lot of models or flowers. That being said, I have made some, and I always toss some gum tex in with fondant and go at it.
Is there an advantage to using actual gumpaste?
Or certain things only GP should be used for?
You can do a LOT with fondant that has a strengthener (I use gum tragacanth powder) mixed in. The only thing I've found I really can't use fondant with is patchwork cutters, I make Mexican Gumpaste for those.
For making figures or doing bas relief or very simple stylised flowers , fondant with tylose in it is fine. However if you want to do finer work and realistic flowers , a flowerpaste or gumpaste is best . I like my petals to be very fine and almost translucent lie real ones .
I also like mexican paste like coral mentioned for structural elements .
a half half mix of fondant and mexican paste also makes a good modelling medium.