Lots of considerations here, and not all will align.
- Toe Overlap was mentioned, and despite the roughly similar frame Reach, the slacker Head Tube Angle on the gravel bikes should get the front wheel further out. Notice the Crux front center is 21mm longer than the S3. I would guess the Revolt to be similar to the the Crux. Considering that, and if you are ok on the S3, I don’t think there will be an issue there.
As to the whole “sizing/fit” side, that is super tricky. True in the general direction that Gravel is following MTB with longer Reach, shorter stems, and slacker head tubes. Couple with that is the steeper seat tube angle in most cases (as seen here). Those aren’t huge amounts, and final saddle position may only shift a tad compared to the slacker road Seat Tube Angle. Considering you went to a zero offset post, those changes align too.
Taking the smaller Revolt, only to add a longer stem is a bit counter to the intentions, but may work fine. If you are aiming to keep the front lower, then the Giant may just be the wrong bike, considering a “smallish” size is taller than your S3, and the next takes that up further.
The Crux is the more “race” oriented gravel bike with the a similar reach, but more drop than your S3. So with the 20mm spacers, and assuming you hit the saddle position the same relative to the BB, this will fit the same as your S3. The key difference will be in handling via the Head Tube Angle, Front Center and longer Wheelbase.
Throwing a dart now, the Crux seems to win, but this is with just about 10 mins review and I would want to look harder if anyone was really handing over $$$ based on my advice.
