This is an accumulative achievement, meaning when you have ran down a total of 175 gang members on one save you will unlock the achievement.
A good tip to running them over is to drive slowly as they will 9 times out of 10 jump out of the way, so when they jump make sure to swerve in there direction and bump them as it does not take much to kill someone with a car. If you go really fast you may be able to hit them aswell, but save yourself some hassle and go slow!
Also, a great place for mowing down gang members is the warehouse outside the Los Muertos run city, out by the the mountains, where there are the jumps inside and a gang leader. You will find lots of open space and lots of gang members!