OpenAI said in Spring that GPT5 was not being trained yet:

https://www.theverge.com/2023/4/14/23683084/openai-gpt-5-rum...

Then hinted a few weeks ago that work had started, but probably not actual training:

https://decrypt.co/206044/gpt-5-openai-development-roadmap-g...

Even if training has started since then, that doesn’t seem like nearly long enough to have it completed by the end of the year, especially if they intend to go through an extended period of testing & aligning work as they did with GPT4 (which they tested for six months before releasing).