What are you referring to with the one gramma looking like a female but acting like a male? I didn't think the sexes were discernible. Do you mean it is smaller like a female but acts like a larger male?
There are actually some morphological and coloration differences I noticed between males and females.
Dominant males have significantly longer ventral fins. Unfortunately, this isn't a reliable indicator as fins often get damaged during capture, shipping, and in fights, and not always regrow fully.
Males also have a more slender body shape.
Females on the other hand have shorter ventral fins, are usually smaller, and have a fuller body shape.
But there seems to be also a subtle color difference:
Males have silver metallic shining scales on their sides in the transition zone between the purple and yellow. My dominant male has this very pronounced while the subordinate male has only a few of those metallic shining scales. The female has none.
The most convincing evidence to me is the behavior. The subordinate male keeps as much distance as possible to the dominant male. Both males demonstrate courtship behavior towards the female, but the dominant male never demonstrates any courtship behavior towards the subordinate male. Whenever they meet the dominant male demonstrates who is the boss, but it never goes to actual fighting.