It's not about Google's model getting better. It is that gpt-5 already has a worse score than Gemini 2.5 Pro had before gpt-5 came out (on the particular metric that determines this bet: Overall Text without Style Control).

https://lmarena.ai/leaderboard/text/overall-no-style-control

That graph is a probability. The fact that it's not 100% reflects the possibility that gpt-5 or someone else will improve enough by the end of the month to beat Gemini.