Does your horse hold his gait well? Any pacing? How old is the gelding in question?
By medium handed, I assume you mean you are working on becoming soft handed and are a beginner? Yes? If no I am sorry but I'm trying!
Okay so why is that important? Well TWH that can consistently hold their gait without extra training can be put in just about any bit. Really, the most common thing I've heard is that the "headset" is better with a shanked bit. I think its baloney. I had a walker, calm, well broke, but only 5 years old. He kept his gait flawlessly. I rode him in a halter, bareback, some days, and he was still a gentlemen. I had a walking horse bit on him, which I regret (I was very young and green and did not know better) but I soon figured out that was way too harsh and worked him down to a hackamore, soft handed. He moved out great in that!
So for you, honestly it may be trial and error. DUMP the wonder bit. Those are NOT for beginners (Personally I don't think they are for anyone). Try a snaffle on him, such as a O or D ring. Plain and simple. Or, if you want something shanked (which you will have to work on your soft hands if you do) then you can go with what the above poster suggested or a similar type.
Personally, I do not believe that walkers need anything different than other horses when it comes to bits. Some will definitely disagree, but the gait and movement are in the training. The bit is simply the tool by which to communicate.
Sorry long winded. Here's the short. Get a soft bit (such as a D or O ring snaffle) and work on softening your hands. If your boy is already well behaved, then it may take some adjusting, but soon you will learn to communicate with soft cues, and your horse will no doubt thank you for it!