Gadot comes from a modeling background rather than an athletic/fitness one. However, she can obviously hit the gym and build up her muscle some more, plus she has been in action films before. Also, the Wonder Woman character comes from another planet and wouldn't need huge muscles to be powerful on Earth. I think people should just wait for the film to come out before judging Affleck & Gadot or the idea of the film as a whole. There still haven't been any pictures or plot released, so for now people should just take a wait and see approach.
The goal is to build to a Justice League movie with numerous other DC superheroes joining the action. You can also expect a solo Wonder Woman movie at some point should 'Batman vs. Superman' be a success...which it probably will be considering you have Batman, Superman and Wonder Woman in a summer action movie together.