"TGA" stands for "Transgender actress." "TG" is short for "transgender," and "TA" means "transsexual actress." In the context of this porn video tag, "tgatas" seems to be a combination of these two terms.
This tag would indicate that the porn video features a transgender or transsexual actress in a sexual or explicit scene. The term is most likely used as a shorthand for easy tagging and categorization of such content. It's essential to note that this tag is specific to adult content meant for an experienced audience only.