How to improve braking with hands on hoods?
Braking is always stronger from the drops than the hoods, as you noted only your lower fingers have mechanical advantage when braking from the hoods, when braking from the drops you have much better mechanical advantage for all your fingers.

That said, when properly set up, you should have no problems locking up both wheels on gravel from the hoods. You should also have no problem locking up the rear on asphalt. 

Ergonomics

There are a lot of factors that can affect brake effectiveness, how you set up your handlebar (ergonomics) can be one major component.  These include:


Brake hood position: if the hoods are angled down, or the brake hoods and bars are angled down, it will be harder to brake effectively from the hoods. You will however be able to brake effectively from the drops.
Handle bar reach: If the reach is set too far it can make braking difficult to get a firm grasp on your brakes from either the hood or drop position.
Bar height - If your handle bars are too low you may also find it hard to brake, raising the bars can improve your reach as well as making it easier to brake from the drops.
Brake reach adjustment: Many road shifter/brake (i.e., brifters) come with reach adjustments which can be used to shorten the reach slightly.
Brake hood ergonomics: Each brand has slightly different ergonomics, and at times slightly different pivot locations for the brake lever. The ergonomics has also changed over time. I personally found mid 2000 Shimano brifters never braked well from the hoods, newer Shimano ergonomic design has improved this greatly, but some of the lower tier models still use the old design and pivot point location. 


Observing some pictures of your set-up and your set-up in use would help