Don't look at the gravel. Look at a clean line between the gravel. Your tires go where you look.
Get off the brake before you get to the gravel. Tires can go over some gravel without slipping, the soft tire rubber just wraps around each individual rock and grips the pavement just fine. Too many rocks too close together, they will slip. Some tires are much better than others at this. Being on the brake makes any tire much more likely to slip, and makes it harder to hook up again once it finds clean pavement.
Generally dual sport tires or knobbies are much better than street tires. New soft tires are better than old hard ones.
How far will the tires slip? Depends on the orientation of the bike to the gravel. Gravel tends to end up in lines roughly parallel to the road. Just like paint stripes or train tracks, if you put your bike where a tire slipping will take you across the stripe or track, if it slips it's no big deal. When the tires gets to the other side of the stripe, it will hook up again. If on the other hand, the tire slips along the stripe, it will never get to clean surface, never will hook up again. Down you go.
Consider every corner to have gravel until you can see it doesn't, that way you won't be surprised.