It's similar to GTA1 but it has some improvements; The graphics are better, there are some lighting effects (if you use dusk) and the game is much more dynamic, with many more pedestrians and gangs walking around and getting into trouble with eachother and interacting with eachother.

You still need X points to progress to the next map and the multiplier still works the same. The main mechanic now is that there are 3 gangs, with each green/yellow/red phones. The respectmeter shows your standing with each gang, killing gang members or doing missions for them affects your standing (like if you kill Yakuza, the Yakuza meter will decrease but Loony respect will improve), your respect dictates which phone missions you have access to and which gangs will shoot at you when they see you.

GTA2 is the last "2D era" topdown game. GTA3 went fully 3D from a third person perspective and it plays very differently. It's starting with GTA3 that the series starting gaining immense popularity.