Big difference was (from someone who knows nothing about GotG outside of the movie) that Gamora is an assassin. Sleek, sexy and dangerous doesn't require you to look strong and physically intimidating.
Wonder Woman is a physical female powerhouse. She should look like she can fuck up more than her hair and make-up.
I'm not saying Snyder won't make Gal look good on camera, but she certainly does not fit the physical expectations of a Wonder Woman at casting. (Her SDCC appearance really didn't seem to change too many opinions either).
The thing about Wonder Woman is that she has been depicted in so many ways she doesn't exactly have a "standard" body type. Outside of her breasts anyway, so if someone reeeeeaaaally wants to they can bitch about that I guess.
To the general public WW's most prominent roles have been the Carter show and the Justice League cartoons. Carter isn't exactly big and WW was pretty small in her animated showings. The only animated showing she had where she looked really imposing was JL: The New Frontier where she was bigger and taller than Superman.
This is how she looked the majority of the time in JLU.