This feature could arrive in the next version of One UI, which is set to debut very soon.
Samsung Galaxy phones and tablets, like most other Android devices, support both of Google's voice assistant services. You can invoke either Google Assistant or Google Gemini on Galaxy devices by saying a hotword like "Hey Google" or by swiping inward from one of the bottom corners. You can't, however, invoke them by pressing and holding on the power button (or "side button" as Samsung calls it). Fortunately, that could change in an upcoming release of One UI.