That's there too; see https://support.google.com/docs/answer/14206696 — you can click on the "Ask Gemini ⟡" and carry on a conversation, e.g. "summarize emails about <topic>" and use those to paste into the doc. (I haven't found all that much use for referencing other files though. But the "proper chat" is useful for saying things like "no actually I meant something more like: …" and carrying on.)