Samsung devices' side button can currently only launch either Bixby or the power off menu when held down. However, this functionality may change with the upcoming One UI 7. According to a recent article, Samsung is considering allowing users to launch Gemini by long pressing the side button. This development could bring about a new level of convenience and flexibility to users. For more information, you can check out the article linked below.
For further details, please refer to the following article:
https://www.androidauthority.com/samsung-long-press-side-button-gemini-3505663/