In my area there are some routes with good tarmac used by cyclists. Those had become regular training routes over the years. However I’m trying to find some new routes (or segments) to add them in the mix. What I’ve noticed is the fact that if I try to stay on good tarmac the ride will easily go over 200km. On the other hand for routes of 100-150km in length there quite a few options but those uses some gravel and bad tarmac roads to connect. This means that a ride will include something like 10-30km of this kind of roads.
Is harmful for a road bike to add those gravel roads on rides?
I hit a interesting section of closed road on my road ride yesterday. Added an element of excitement… especially when I got to a section where a small river was running over the road.
I run ~25s on my bike so not the best for “gravel” but can get it done if need be. Smooth dirt roads are wonderful though. The biggest thing I’d worry about on NASTY roads is flatting. The frame can handle it unless you manage to get a rock really wedged somewhere bad.
Things to consider will be:
What kind of rims you are running
What kind of tires you are running
What pressure those tires are running at
Do you have a spare onboard incase s*** goes south
It will work fine.
I am going after some of the local gravel KOMs on my Specialized Venge with 28mm GP5000 TL tires 
Just realize loose gravel sucks power, so count on slower speeds and more effort for those sections. If you add headwinds and hills, riding on loose gravel will feel like crawling.
Adding some gravel, or just dirt roads, brings such a large amount of joy to my cycling. Just feels so free to ride wherever I bet, rather than contained to a road. Kinda like being a kid again
What kind of wheels/tires are you riding?
In general, Id feel much safer doing this on tubeless than with tubes in my wheels.
The wheels are Fulcrum Racing 7 with Vittoria Zaffiro Pro G2.0 Graphene 700x25C tyres. I did something like 1500-2000km with those tyres so far.
Is harmful for a road bike to add those gravel roads on rides?
How precious is your paint to you? You don’t mind fucking it up with a few nicks or scratches, do ya?
Go for it and have fun
Besides what others said, it depends on the type of gravel you have. Near me its all crushed limestone. On good days it is no different than a paved road. On bad days, you want 3 inch wheels. If it is extremely dry out, expect gravel dust everywhere. Other parts of the country, the gravel can be the size of golf balls, sharper and give you more punctures.