Although these compounds look similar, they are fundamentally different materials on a molecular level. PET is the result of two monomers combining. PETG may involve these same monomers, but also includes glycol, giving it different chemical properties than PET.

PETG is less brittle and more pliable than its PET counterpart, making it ideal for bottles. It also has better shock resistance and is better suited to higher temperatures. （from https://www.acmeplastics.com/)