Capture Johnny Gun - GTA Online: The Contract 

Get into the car, and move toward the location.

You've to go to the Johnny Guns' last known location.

You'll not have his precise location, but once you enter this door, you'll find Johnny Gun easily.

Now, you'll have to kill the hired gunmen.

Kill all of them, Johnny Gun is escaping on car.

Use Grenades to kill them.

Now, after killing them, get on this car and follow Johnny Gun.

Now, you've to go toward LSIA.

After reaching the airport, Enter the hangar via the side door.

Now, you've to find Johnny Guns in the hanger.

Kill all the men on your way.

Johnny Gun, will be standing on top, shoot at him.

Now, go upstairs to capture Johnny Gun.

Now, take him outside.

After that, take Dr. Dre to the Pacific Bluffs Country Club.

Park the car on the yellow spot.

After that the mission will be accomplished.