I use one of these;
You don't need a grease gun. Just put it on the ground and stand on it. (that pushing it down with your hand doesn't work very well.)
BUT, I use it in conjunction with Bearing Buddies. I like Bearing Buddies, as long as you don't go crazy with using them, and use them properly. I totally repack my bearings every 2 years. And in the off years I use the Bearing Buddies. 2 shots a wheel in March and 2 in August.
I've heard of some folks using BB and juicing them every time they use the trailer...EEEWWWW.
I've even seen some people use BB with BRAKES...WWHHHEEE.