PETG (as the end product) doesn't actually contain glycol, the G indicates that it's manufactured using a modified glycol mixture. All the glycol is used in the reaction that forms the polymer, there's no free glycol in the final polymer (or at least it's an insignificant amount present only as a contaminant).

PET is a polymer produced by the esterification and condensation of terephthalic acid with ethylene glycol.

PETG is a polymer produced by the esterification and condensation of terephthalic acid with a mixture of ethylene glycol and another glycol (unspecified but often (cyclohexane-1,4-diyl)dimethanol or trimethylethyl glycol).

PET-T is a polymer produced by the esterification and condensation of terephthalic acid with a mixture of ethylene glycol and trimethylethyl glycol.