How do you develop a one-pager for analyst firms like Gartner to convince them about your product?

I don't know that I would recommend using a one-pager. In my experience, analysts are not going to read a document you send them unsolicited. The best approach would be to schedule some inquiries and briefings with the analysts as I've described in other answers. Then once you've had your briefing or inquiry, you can follow up and send them documents that relate to what you've discussed. This could take the form of one-pagers. The most important thing to include on any documents you send analysts is customer proof points. Highlight a customer story in everything you send them, even if what you are sending them is not customer-related. You can always squeeze in a customer quote, result, or story alongside whatever other content you are including in your one-pager or deck. They want to see that people are actually using your product and seeing meaningful results. That, more than any specific structure, is what I would recommend for any document you share with an analyst.