Hard Stops and Encoders 

For motors that have hard stops for the open and close limits or have a close stop and encoder (motor rotation counter) for the open limit you normally need to run a learn cycle from the control board that cycles the gate open and closed to find out how far the gate travels so during normal use if the gate is obstructed by something like a car parked it knows to reverse rather than stop. Also if a motor has soft stop this enables it to slow down before it reaches a stop so it doesn't bang loudly.

Gates that use hard stops only required a stop at the open and close position whereas the more accurate encoder motor requires only a hard stop for the close limit or as a reference in the middle of travel somewhere.  With sliding gates it's a good idea to have an open hard stop regardless in case the gate is on manual to prevent it being push open too far and going off the end of its guide and falling over, this can be dangerous.

Limit Switches 

Limits switches can be used to set the open and close positions without the use of any hard stops, although as mentioned above with sliding gates hard stops are necessary for safety reasons. 

Typical limit switch arrangement for an articulated arm swing gate motor 

If the Motor has soft stop then you may need to either run a set up routine from the control board to learn the time it takes to open or with some motors the limit switches set the point that the motor begins to slow down with the actual close position being a pre-set time after this.