I did Gravel metric and it had some rough terrains and gravels and some grassy areas. Thank goodness there weren't too many hills.
I rode a road bike designed for wider wheels/tires and I used 700cx35 gravel grinder and I was fine with it.
Personally, I'd recommend to people to ride the bike they're most comfortable with, in terms of handling and comfortability for a long bumpy ride.
There were people who rode a Brompton and they both regretted the decision - well, one of them did, the other was a Brompton rep.
A lot of people rode a hybrid mountain bike converted with road handle bar, with no suspensions - and they enjoyed that.
So, I really do believe that you should ride whatever you'd be most comfortable with.
I did the Rough Road 100 and rode with Will on the Gravel Metric. Rough Road was more pavement than I expected, maybe 1/3 of the course. The gravel portions were basically flat fairly smooth farm roads where you pretty much rode in tire tracks for a smooth line. There was also a 3-4 mile or so stretch of really rough doubletrack that was just bone jarring and annoying as opposed to being a challenge to relish. Now Sager Rd at Barry, that was a challenge!
Gravel Metric was a lot of fun! The course was a mix of gravel farm roads, grassy snowmobile trails, and a few sections of what I think were private farm roads that pretty much only saw tractors. There was also a shallow stream crossing that we were able to slowly ride through. However the headwind was brutal when the course turned east for the final 15 mile leg. Then pouring rain with 10 to go! Regardless I thought it was a lot of fun and will be back next year if I don't do the Badlands Gravel Battle up in ND.
For both of those I rode a cross bike with 700x38 Challenge Gravel Grinders. I feel like it was a good tire and bike for both events. Some people had mountain bikes, others had cross bikes with narrower tires.
I've done the 24 miles at Barry Roubaix for three years and every year I say I am not going to do it again but I keep coming back. It's a cold ride and the weather and conditions have been different every single year. I ride my cx bike. Lots of hills, sometimes ice, sometimes sponge-like mud. Always so happy when I cross the finish line (knowing the suffering is over).
Palos is a great place to try out your bike - there's a multi-purpose trail that has some gravel, hills, etc. I also did some of the easier MTB on my CX bike there and it was a lot of fun.
You never know what to expect. If you do any of the gravel rides, please do tell us how it goes. Good luck!