I would think that 32mm GP5000 would be a good option, but I've never used them.

The conti terra speed is a fast gravel tire that I've ridden quite a bit on road and gravel. The 35c tire runs narrow on my rims (33mm). I'm sure the GP5k is faster, but the terra speed wouldn't be a bad option.

Is the finish on gravel or tarmac? Any steep gravel climbs where traction will be an issue? Any 90 degree turns on gravel?

I run 32mm GP5000 on my road wheels and they are fine on good quality gravel especially if your are just going along in an essentially straight line. You could look at the Gravelking (smooth/file tread) if you will be cornering on gravel more. I ran these and found they gave traction in all conditions except really wet mud/grass.

Also to consider: Do you think the gravel section will be the defining point of the race? (i.e., breakaways/attacks are likely to happen here) or will it happen on the road?

i run zipp 303s with 32mm GP5000s tubeless and they are a phenomenal ride. Most all my riding is pavement so cant really speak to gravel quality but i run them at appx 55-60psi (200lb rider weight) and have no issues keeping up with the A group weekly rides... Plus i guarantee the comfort of my ride is top quality!

32C 5000 is the choice. You could get away with 28's probably.
I did a gravel race once with 50 miles of gravel on 23C GP4000 with latex tubes!

You need a wider tire if the gravel is soft and sandy (for less sinkage), or it's really bumpy (for comfort and flat protection).

I'm a fan of the GP5000TL ride quality but found they wore a little fast and that made me question their durability. (damn, they road nice though!) I prefer the Specialized Roubaix Pro as it has simply been bulletproof! I run the 30/32 size that measures close to 34mm on my 24mm internal rims (Mercury G1). These are great training tires for the road and have handled everything I've thrown at them off-road. And when I say off-road, I mean legit singletrack/XC trails in addition to run-of-the-mill fire roads.

if your bike will fit it, I love the Rene Herse (previously Compass) Tires "Snoqualmie Pass." 44mm of cush, but it rolls VERY fast.

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"Non est ad astra mollis e terris via." - Seneca | rappstar.com | FB - Rappstar Racing | IG - @jordanrapp

A friend of mine won the DAMn (240 mile gravel race across Minnesota) this year on GP5000's in 32. There's no question that's the absolute ultimate in performance for tarmac and gravel. The question is how comfortable do you want to be? I rode Gravel King slicks 38mm in the race that day and sure was comfortable, but it sucked getting my ass kicked to someone with a lot faster tires!

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24 Hour World TT Champs-American record holder
Fat Bike Worlds - Race Director
Insta: chris.s.apex

I see stuff like this and wonder if my desire for a new road bike that fits 32C tires will double as a gravel racer for most conditions.

I use my Domane for most gravel with tires up to 40.

If I want to go crazy fast the Madone is a rocket ship on gravel. :)

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24 Hour World TT Champs-American record holder
Fat Bike Worlds - Race Director
Insta: chris.s.apex

IMO- It really depends upon the course and your handling skills relative to whom you are competing against. I've seen pure roadies disappear on gravel sections on fast tires because their confidence in their tires/skill level let others roll away. If you are comfortable on the 5000's then they would be fast.

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I have deceptive speed.........I'm slower than I look!

I want to win the race :).

Same. I’ve been in a couple gravel races where roadies just “road” away from the field on their road bikes because they were good bike handlers and there weren’t enough rough gravel sections to pull them back to the field.