https://en.wikipedia.org/wiki/Roundup_Ready

The crops are designed to withstand Roundup, while weeds are not, so farmers can use Roundup to control weeds with crops that would normally be killed by the herbicide.

It is important to know what is done and where the GMO is used: In some container or open air or open water.

(I'm sort of in biotech.) This is one way to look at it. But I'm pretty sure one of the mechanisms that allow the plants to survive being covered in certain pesticides also allows them to effectively decontaminate / degrade the pesticide.

A plant engineered to express an enzyme capable of degrading organophosphates [0] could allow for both the plant to be protected from exposure to the toxin & even after harvesting still express low levels of the enzyme which should clean the plant.

That's the idea at least... From experience, it's not nearly that simple. E.g., the degradation products are also somewhat toxic.

[0] https://en.wikipedia.org/wiki/Aryldialkylphosphatase

If this is the case then non GMOs sounds more appealing.