Has anyone managed to create a custom Gem in Gemini for image generation that actually works well?

I’ve tried a lot of approaches: giving it JSON instructions, adding reference images, and describing the style in detail, but the results still don’t follow the intended look consistently.

What I’m trying to achieve is simple:
I want to upload one image and have Gemini generate a new image in the same visual style.

At the moment, it doesn’t seem to handle that reliably.