LLMs are autoregressive, so they can't be (multi-modality) integrated with diffusion image models, only with autoregressive image models (which generate an image via image tokens). Historically those had lower image fidelity than diffusion models. OpenAI now seems to have solved this problem somehow. More than that, they appear far ahead of any available diffusion model, including Midjourney and Imagen 3.
Gemini "integrates" Imagen 3 (a diffusion model) only via a tool that Gemini calls internally with the relevant prompt. So it's not a true multimodal integration, as it doesn't benefit from the advanced prompt understanding of the LLM.
Edit: Apparently Gemini also has an experimental native image generation ability.