Whether up/down the distance will be shorter for the bullet.
Therefore if the angle is +/- 30 degrees the real distance the bullet flies is 0.9 of the range indicated. Ie 100m at a 30o angle is the same as shooting at a target at 90 m
At 45o it would be 0.7 ie 70m.
These are good rules of thumb If the angle is less than 20o and the range less than 200 dont worry about the calculations
The maximum effect that gravity has on a bullets flight is when its flying parallel to the ground.
Your shot will hit high if you shoot up or down hill at the same distance. this is lessened though at closer ranges were the bullet is still at very high velocity when the bullet is travelling "point blank"