One gatefold in magazine

There's several ways to achieve this result. The way you have it set up is probably OK but always check with your printer to find out what they prefer. You have it setup as two separate pages where technically it's a single larger page folded in half. To get really technical, that fold out page needs to be shorter than the page it's connected to. Again, probably more info than you are asking for. I've attached an updated image that I commented on to answer the questions you asked in your comments. To answer your question, it is the right setup, the real question is whether your printer wants it as a separate page or a single larger page. To do the single larger page, you can select the Page tool and make the page larger to accommodate the foldout. In that case you'd need to delete page 26 and 27. I hope that makes sense.