How about wider tires for your Gran Fondo? Sounds like it can fit up to 35c tires when equipped with disc brakes:
https://www.racycles.com/product/detail/8035
Specialized came out with the Diverge, pretty much a Roubaix (similar to your Gran Fondo) with more tire clearance and slightly beefed up:
https://www.specialized.com/us/en/bikes/road/diverge
I did my 20 mile Tour de Plutonium ride the other day on my Roubaix:
Sometimes I'll do that ride on my Roubaix, sometimes on my FS XC bike. It really depends on my mood and if I'm mixing in roads and bike paths, or a little bit of single track. On the gravel path, either bike is fine.
One thing to be aware of is the road bike with 25c tires needs a lot more attention and skill. Even on a groomed gravel path there are sandy and washed out sections that will take you down quickly. A mountain bike tire will just float over those. A 32c cross tire helps a little, but not a ton. You still need to be on your game.
I'd love a cheap hardtail 29er to fill that spot in my quiver. It's just hard to justify when I have two perfectly fine bikes already. Kinda like I want a 100mm ski when I already have perfectly good 85, 90 and 110mm skis. How much would I really use it? Yes, I'm aware of the OP's proclivities