Before you invest in a smoker, learn whether you like smoking.
You can do it on your gas grill. Just put the meat at one end of the grill, and use the burner at the opposite end, on low. Pre-soak your wood chips of choice, and put them in a folded foil packet with a few fork holes poked in it, over the burner.
When I had to move into an apartment in Georgia, and wood and charcoal burning grills were prohibited, I used this technique to make every kind of BBQ you can imagine, from pulled pork that was smoked 16 hours, to fall off the bone baby back ribs. Honestly, it's nearly impossible to tell the difference between this method and a proper smoker - I've even had people accuse me of using liquid smoke on my ribs, because they were convinced a gas grill could never make such delicious smoky meat.
Just make sure, if you're going to do a long smoking like pork butt, that you know your propane tank has plenty of gas, or that you have a spare tank handy.