>gemini -p "Say hello"

  Says hello, and just returns right away.

The gemini doc for -p says "Prompt. Appended to input on stdin (if any)." So it doesn't follow the doc.

gemini "Say hello"

  Fails as it doesn't take any arguments.

For comparison, claude lets you pass the prompt as a positional argument, but it does append it to the prompt and then gives you a running session. That's what I'd want for my use-case.