
In the author's world gammas are the secondary gender that is the least affected by pheromones. They often work as bodyguards for extreme alphas, because the chances to mutate them is very low. That doesn't mean it's impossible, but they have to be under the influence of a lot of pheromones for a long time and the chances they survive is low. Also their health is affected, they will become weak, can't walk anymote and their lifespan shortens. If they give birth to a child it will only get worse.
Gammas are rare.
Ashley Dawson, who gave birth to Chase's father was a gamma who was mutated to an omega. Novel title is "Eat me up if you can".
In omegaverse what's a gamma?
Edit:
Found answers through Google and reddit.
https://www.reddit.com/r/FanFiction/comments/1fg8ijf/omegaverse_what_are_gammas/