Falling down a hill.

sliding

// Gravity pulls the player down,
// but the slope of the hill pushes them in another direction.

static const float G = ...; // Acceleration due to gravity.

// First find what percentage of gravity is wasted opposing the ground normal:
float dotN = Vector3.Dot(groundNormal, -Vector3.up);
// Or simplified, just -groundNormal.y

// Subtract that from the down vector to get the percent
// That pushes the player along the ground plane.
Vector3 tangent = -Vector3.up - groundNormal*dotN;

// If you just want an uphill/downhill value, then dot with the player's forward direction.
float dotForward = Vector3.Dot(player.forward, tangent);

// Multiply that value by some constant to get their acceleration/deceleration.
Vector3 accel = ACCEL*dotForward*player.forward;

// Multiply dotN by some percent and subtract that for friction.
accel -= FRICTION*dotN*player.forward;

// Multiply their current velocity by some coefficient and subtract for drag.
accel -= DRAG*player.velocity.magnitude*player.forward;