TL is the abbreviation of total length, which can refer to the length of clothes, skirts and trousers, etc.

G and x are variables used to calculate the length of parts, because they are not fixed reference characters in clothing parts (reference characters in some parts of clothing are fixed, such as BP point, which refers to the chest height, and HSP is the shoulder and neck point. But here g and x should be a variable, depending on how it is marked in this book or this model, so you should read how it is marked in the book.