I've not seen an acronym PETG but PETE is common in the US and PET in the most other parts of the world.
PET bottles are very suitable for all common photographic solutions. Watch out for the fit of the cap. Good quality cap should be used. Good ones have soft teflon lining to ensure good seal.
The biggest weakness of PET is heat. It can't take high temp. If you mix water and NaOH in a PET bottle, it can melt and make a hole. But as long as hot filling is avoided, PET is good for darkroom chemicals.
Incidentally, recycled PET is hard to sterilize without going through troubles. This is why the reuse and recycle rates of PET have been lower than other plastics.