I’ve done a couple Grinduros; here’s what I have observed.
1: There’s typically only one climb and it will be 1-2 miles in length.
2: There will be a flat/rolling segment.
3: Typically there are two downhill segments; one fast (fire roads) and one technical (single track).
To be competitive on the segments you have to pedal, you’re looking at 6 to 12 minute efforts.
I suppose it may vary by location but from what I’ve seen, they all appear to be pretty similar; VO2 barf fest length.
The downhills are really worth paying attention to as well. They can be pretty long and if you’re not a good descender or are underbiked for the terrain, you could potentially lose a lot of time.
I did my first Grinduro on a gravel bike and switched to a hard tail MTB for the next time. The hardtail was a much better choice. The only segment it was noticeably was the rolling hills/flat segment where faster rolling tires and more optimal gearing/position could have shaved some time off but for the steep, loose climb, I was able to get in a much better cadence with too much weight penalty and on the downhill segments I was significantly faster.
This year I will be on a gravel bike with 2.2"x27.5 tires and I think that may be the sweet spot. We’ll see.
TL;DR - No segment will be more than 15 minutes (hopefully!), it’s rowdy enough to penalize a pure gravel bike unless you have MVDP level handling.