Another fact to consider there is that Ben Affleck is older, and a man. He has lines and character to his face that a younger actor or a female actor simply may not have. They are two very different people. Trying to compare their acting ability based on a still photo is just silly. Nevermind that Affleck has been acting since he was what...8 or 9?
A lot of the complaints here seem to be that Gal has certain "set" expressions and a "blank face". That "blank face" is part of the reason she is considered beautiful. She has very few facial flaws, blemishes, etc.
Here's the thing...people are born with a certain bone and facial structure, and even with surgery, can only do so much to alter it.
Not everyone has the craggy, extremely expressive face or particular type of facial structure that is often mistaken for great emoting, when it fact it's just a face with a lot of character in the first place. I see this all the time when people are judging actors, even locally. An actor literally sits there doing nothing but looking attractive, and people fall all over their amazing performance, when in fact they didn't do all that much. There's a reason that both Broadway and Hollywood are full of people with certain types of facial structures.
Watch her performance closely. Look at her eyes, at the softening or tensing of her jaw, at her body language, and very small elements of her facial performance. The movement of the eyebrows, the movement of the eyes themselves...that's acting. It's subtle, but it's there.
No, she's not the best actress who ever walked the Earth. She just started doing it, in many respects, and it would be unfair to expect otherwise. But she's certainly capable, has made solid improvements in her performances in the span of really just a few years, has varied her portfolio considerably (she's not exactly been typecast, she's been in dramas, action films and comedies) and she has sold the types of moments Wonder Woman needs to have very effectively thus far.
And I hate to keep coming back to this, but do you REALLY think WB would just cast an attractive woman who can't play the character and has to carry not only this film, but Justice League movies? Hollywood is full of attractive, exotic women who could look the part and even handle much of the action. There's something special about Gal Gadot, a specific combination of vulnerability, intensity, charm and physical capability, and pretty much everyone who has worked with her has said as much.