it IS acceleration due to gravity!! the only reason you roll down a slope is because of gravity!!!
when you're standing on a flat piece of ground, the gravity vector points straight down, which is equally and oppositely opposed by the normal force.
now, when you're standing on a slope, the gravity still points straight down toward the center of the earth, but if your frame of reference is with the road as being horizontal, then now your gravity vector has TWO components - one that pulls you forward in the downward direction of the slope, and another that pulls you vertically down into the asphalt itself - which is equally and oppositely opposed by this new, lighter normal force. in fact, the asphalt "feels" like you are lighter because it does not have to support your full weight - it only needs to support that component of your weight which is pushing you vertically into the asphalt. if you take the extreme of a "slope" which just goes vertically straight down, the asphalt doesn't provide ANY normal force at all, and 100% of the force of gravity is being used to pull you "forward" down the "slope".
You were correct that ACCELERATION due to gravity is constant, no matter what. But two things you neglect:
1. 9.8m/s^2 assumes that the object is free falling in a VACUUM, with no opportunity for viscous drag in the equation. obviously aero drag is VERY important in cycling, so there are other kinematic components to take into account when discussing the above problem. your reference to the acceleration equation while correct, refers to a specific problem - that is, an object in FREE fall with NEGLIGIBLE frictional forces. A bike going down a slope is far from free fall - and there is plenty of friction involved from the rolling resistance of the tires to the rolling resistance at the hubs to the aero resistance of the bike and rider.
2. althought acceleration is constant, the FORCE DUE TO GRAVITY depends upon the mass of the object. Gravity pulls a 200kg man down to the earth with twice the force that it pulls a 100kg man.