People fail to consider that Gal was pregnant at the time of shooting of Justice League, she resumed re-shoots just few months after delivering a baby (which is really difficult for some actresses), how could she train then ?
Now she is training under same trainer who trained Vikander. (her earlier trainer was Mark Twight, who trained Cavill.)
But, the fact that everybody's natural physique is different and it responds differently to training should be taken into account and the fact that Gal is naturally lean, so she finds it difficult to gain muscle.
Then there's the fact that Vikander is 5' 6" and Gal is 5' 10 ", Gal being much taller, she has to eat a lot more and train a lot more in order to get visible muscle definition.
Having said that, I'm sure Gal will do her best and her trainer will try for that.