Recent 17+ Thora Birch Walking Dead Gamma. After taking a break from acting, birch resumed her career in 2016 and has since starred in several independent films.4 birch also played whisperer mary / gamma on season 10 of the walking dead the walking dead. What can you tell us about what is coming up with gamma in the future here on the walking dead? So while the walking dead hasn't shown gamma before, fans can deduce she is important to the whisperers' leadership hierarchy.