GEMINI.md vs AGENTS.md: do you need both?

No. Keep your instructions in AGENTS.md and point Gemini CLI at it: list it in context.fileName, or make GEMINI.md one line, @AGENTS.md.

How do they differ?

Where: GEMINI.md: ~/.gemini/GEMINI.md, the workspace and its parent directories, and a directory's own file once a tool reaches it.; AGENTS.md: The repo root, and any subdirectory, depending on the agent.
Imports: GEMINI.md: @file.md, relative or absolute, up to 5 levels deep.; AGENTS.md: None; it's plain markdown.
Change the file name: GEMINI.md: context.fileName in settings.json takes one name or a list.; AGENTS.md: Fixed by each agent.
Repos with one: GEMINI.md: 8 of 401 (2%); AGENTS.md: 203 of 401 (51%)
Agents that read it: GEMINI.md: 1 of 9: Gemini CLI; AGENTS.md: 6 of 9; which ones

7 of the 8 repos with GEMINI.md also have an AGENTS.md or CLAUDE.md. Empty files don't count.

Keep the instructions in AGENTS.md.
Commit .gemini/settings.json with the setting below; project settings override a user's.
Or add a GEMINI.md containing @AGENTS.md, which also passes our check.
Run /memory show in Gemini CLI to see what it loaded.

Gemini CLI · .gemini/settings.json

{
  "context": {
    "fileName": [
      "AGENTS.md",
      "GEMINI.md"
    ]
  }
}