As you are probably aware there are many types of gravel/dirt roads. Well graded and reasonably well packed roads are no problem for a Ninja 250. My first motorcycle road ride on my Ninja I took it 1/2 mile down a dirt road and it felt very little different from a road ride.
However if the roads are deep loose gravel or even worse loose large rocks then a slick tired bike will be a handful. The Ninja is probably better than most due to its light weight.
As long as its actually a maintained road and you take it slow durability and suspension should not be a problem. Only if you are riding fast on rough roads or bottoming out the suspension would you have a significant issue.
If you don’t mind spending a little more the WR250R would be a great bike for your situation. About the same power and efficiency as the ninja 250 but lighter and better suspension. Should still be capable of longer commutes without a problem but more fun on gravel and capable of off road.