I've been riding more gravel roads this past year and have been using my touring bike (Surly Disc Trucker) ... steel frame and quite heavy what with racks and fenders.

Couple of thoughts to consider:
1) Sounds like you've done 100k on pavement and survived. Have you ever done consecutive days of long rides? Especially as we get older (I'm about to turn 65), recovery time can take a bit longer. If you haven't already, may not hurt to try riding a couple of 100k rides back to back to see how you feel.
2) Find out what the course really looks like. I've joined a few "gravel grinder" or "dirt" rides and there can be a big difference in what the course looks like. As noted by another responder, if the course is largely hard-pack, well-maintained roads, you s/b fine (assuming you're able to manage 3 consecutive 100k days on the bike). If, on the other hand, by "dirt" they mean the whole range of loose gravel, single-track, unmaintained fire roads, then that can be a whole different story. Even with the optimal bike and tires, 100k can feel like 100 miles if the road/trail conditions are sketchy. From personal experience, I joined a gravel ride that ended up having miles of loose sand, off-road trail riding, a stream crossing, and more :-) ... out of my league in terms of skill and the bike I had and it was physically draining for me.

Good luck ... keep us posted on what you decide and how it goes.