In reply to supersteve:
I'm no expert, but I think the width of your rims on your gravel bike may limit how narrow you can go anyway - having said that I think it was my 32 mm Marathon Plus tires that I put on my gravel bike that came with 38 as stock, although it might have been my Marathon Plus Tours which are 35 mm. I definitely used the 32 mm ones on my old CX bike for several years, but I'm not sure if they were a narrower rim.
A lot of modern real world and lab testing seems to show that narrower tyres don't make such a massive difference anyway, and they will be hard on your bum if you're used to nice big fat cushty gravel tires! I bet a road-focused slick tyre at 32 or 35 will be just as fast as 25 or 28 and more comfy on a long ride.