I agree with what's already been said, and don't have much to add to it.
Although I don't have any personal experience with Gnomenreigen, it's generally considered more approachable than the Transcendentals, the Paganinis or the Trois Etudes de Concert (and a suitable entrée to Liszt's etudes for that reason).
I'm aware, too, that there are diverse opinions about the technical challenge of Chopin's 25/12. It's one of the harder ones for some people, while others apparently find it significantly less formidable.
I concur that F-I is the least demanding of these three pieces (and by a fair margin, I would add). It would be easier for me to play F-I well than to learn the others even to a lower standard, and that's probably true for many other people as well.