Least Gate is the city gate which stands nearest due Hubwards on the city walls. While the destination is not marked on The Streets of Ankh-Morpork and the scale of The Discworld Mapp makes it unclear, it would appear to be the road to one of the Stos, most possibly Sto Kerrig. It opens out into Pallant Street, which appears to follow the lowest contour of the valley between the Tump and Nap Hill.
The illustration on pp2-3 of The World of Poo purports to be of the approach to the City from outside via the Least Gate. However, the artwork is somewhat idealised and has cabbage fields (and attendant rabbits) growing suspiciously close to the city walls, where the reality is most likely to be an overspill suburb from an overcrowded City. The skyline of the City should also be seen to rise to left and right of the Gate, to the Tump and Nap Hills respectively.
The revised and expanded city map contained in The Compleat Ankh-Morpork has a handy little panel illustrating how all roads lead to Ankh-Morpork, even if you happen to be leaving the city and walking in the opposite direction. (In this case out of Least Gate, where the road is confirmed to lead to Ankh-Morpork via Sto Kerrig.)..