Though you can use AA cannons or regular unguided rockets (make sure they're at least level 3 before you attempt to minimize the amount of hits you need to make on them), but I recommend using the Killer Bee while on D-Horse as you can lock on, fire, sprint away, and repeat until the gunship is dead.
I recommend doing this on missions 18 or 19 (there are others but these two are the freshest in my mind) as they both have a gunship immediately upon arrival. Kill it, reload checkpoint, repeat. Plus if you die you don't have to worry about a long stretch of land to cover before getting to the gunship again.
EDIT: I found it was much easier loading up mission 12: Hellbound. Just load it up with either a Grade 3 (beginning rank) FB MR R-Launcher, or a Grade 3 GROM-11 launcher. The first two of my gunship kills took ten minutes in the first method I have above, but the final three took a total of 5 minutes. Just go with this one instead of 18 or 19 as the Killer Bee's range is dreadful. You'll need one hit with either of the other two launchers to kill the gunships.