Under a very strict interpretation, that should mean any LLM trained on GPL code should be GPL as well. To prove that is the case seems tough though, just like artists you would need to make the LLM produce a substantial part of the licensed work to prove said work was part of the training data.

If that would hold up in court is a completely different question though, and then there is also the question of what organization is willing and able to cough up the legal fees to litigate this.