For maximum top speed you should reach the top speed at the same time as your engine is producing the maximum power. If you do that then it’s physically impossible to go any faster. So check the engine speed on your next top speed run and see how it compares to the power curve of the car. If you have gone past max power then you need to make the gear a little taller. If you haven’t reached max power yet, then you have to make it a little shorter.
And yes, peaky means that the power rises (and drops) relatively quickly. Typically you will see this in engines with low torque and high power. Engines with high torque have less peaky power curves generally speaking. Less peaky engines also benefit from more closely spaced gears, but they don’t benefit as much as the peaky engines. Basically, the flatter the power curve, the less they will benefit. I think it was in GT6 where you could use the power limiter to create engines with completely flat power curves. The interesting thing about that is that then you could set whatever gear ratios you wanted and it wouldn’t make any difference to the lap times, because the engine would pretty much always operate at peak power no matter what you did to the gearbox.
As a general rule it’s better to have the gears get progressively closer as you go up the gears. There are two reasons why your car accelerates faster at low speeds than at high speeds. The first reason is that aerodynamic drag increases, the second reason is that force = power/speed, so the faster you go the less force can be produced at the wheels.
Do a lap around a track you want to tune your car for and see what kind of speeds you’re mostly running at. If you spend most time between 150 and 250 km/h, then the optimal gearbox is the one where the gears are closely spaced within that range of speeds (but not too close, because it takes time to shift gears…)