In my experience with Gammas, they often come with their own flavor of sensual allure - like the Betas. But Gamma types, being far more independent, will lack that Fe charisma and persuasive charm. Especially if they're female, they may be more cat-ish, and a little aloof (yet very sensitive/frail somehow). One way I can describe this is by a sort of "Megan Fox" look. Often times the eyebrows will be raised high, the eyes (like those of Beta types) will be focused, intense and sharp.
But unlike the Betas, their mouths and smiles will not be "smooth" and wide open to the sides. It is usually pinched around the nose area, and creates a look like:
Rather than coming across as 'confrontational' like the Beta's more horizontal mouth-muscle tone, Gamma types can look more sassy and 'upset' in a passive-aggressive way even when they're not. They have a partial 'snarl' going on a lot of the time.
Their laughs may be unordinarily loud, and their facial expressions can be very intense or even scary-looking unintentionally. When they talk it's a little bit dismissive, and when Te is strong... their speech can be very fast, snippy (with a lot of head rattlings) and matter-of-fact.
When Fi is strong in the Gamma, then the voice sounds whispery like the voice of FiSe Michael Jackson and FiSe Marilyn Monroe.
I wonder... how do these ideas align with your own observations of gammas?