I hate gain maps in JPEGs. I noticed that because I insisted on capturing JPEGs and not HEIF on my iPhone, and I found that the iPhone would also show overexposed parts (such as the sky) brighter than plain white. That's how I came to know gain maps. Fortunately on iOS, although I cannot disable the inclusion of gain maps in photos I take with Camera.app, I can stop Photos.app from rendering them: just turn off "View Full HDR" in the settings.

The Metapho app on iOS seems to be a good app to view the gain map itself. I give it no access to my photos and use the share sheet to view gain maps of specific images.