I have two Gizmos and both have the same issue with rear wheel bearings. I assume that any Gizmo with a belt drive would have the same issue. I believe that due to the tension required in the belt, the weight on the rear wheel, loads due to acceleration and regen, road shock, and the narrow R12 bearing outer surface the aluminum cannot handle the pressure. I believe that it actually cold flows slightly allowing the bearing to start to move and continually rounds out the hub. On both Gizmo hubs the inner seal was warn down to the metal support due to the bearing slopping around as the wheel turned. It also caused a lot of noise.
As a proof of concept I took a pop can and cut a strip of aluminum just long enough and wide enough to slip into the groove made by the bearing. This worked just fine for about 50 miles and showed me that the noise was caused by the bearing movement. (Ok, for you physics types like me it was actually hub movement on the bearing but relative movement none the less.) Since this worked I decided to use a harder material so I bought some brass sheet and repeated the process. On the second try I finally got the piece made to fit and everything was fine for a while. Then one day I noticed that the outer seal was working out of the hub. Later it was even further out. Finally I took the wheel off and the seal just pulled out easily. What I found was that the brass sheet was working its way out from around the bearing and pushing out the seal. Time for a different approach.
Since the inner, belt side, wheel bearing also wears out quite fast I thought I would like to install a tapered roller bearing on each side and gain an adjustable bearing, more bearing surface, a wider outer race surface, and longer bearing life at the expense of a little more friction. The problem I ran into was that I couldn't find a bearing with the dimensions I needed. The aluminum hub would have to be machined out and the aluminum may not have enough strength to keep the outer race compressed. I confirmed this with a couple of knowledgeable bearing people. I then went to Waite Specialty Machine Inc. in Longview, WA and talked to Keith Warring. Keith is an awesome guy and knows his stuff. I took in one of the hubs to show him what the issue was. I explained what I had tried and what I had found about bearing options. He suggested that a steel sleeve be inserted and that two bearings be installed on the belt side. They regularly sleeve motors and other applications where the bearing seat has rounded out too large to properly hold the bearing. For these types of applications they machine the hole just slightly smaller than the sleeve then they heat the outer piece and freeze the sleeve and insert them. When the two are at the same temperature they create an interference fit where the sleeve won't move. They also cut and faced the spacer that goes between the bearings and installed the bearings so the hub was ready to go.
Note the extra piece between the bearing outer race and the hub. The picture below shows a different angle where you can see the sleeve.
The outer bearing where the castle nut goes did not show any signs of wear. In fact, the inner wheel bearing on all three wheels seems to be the first one to go. I believe this could be due to the fact that the shaft flexes a fair amount so those bearings don't get the same road shock.
The cost of all of this was a little over $500 for two hubs. As with much machining the setup cost is quite high so doing two hubs at once was much cheaper than doing one at a time.
The other issue I've had with Gizmo bearings is that the inner seal seat, the collar on the shaft, had rusted and then wears out the seal rapidly. I was able to sand down the collar, the metal is quite soft, and install a speedy sleeve which is a stainless steel sleeve that goes over the seal race to gives a good surface for the seal to ride on. These are not cheap! One cost me over $40 but I'm sure it was less than replacing the axle or the entire rear swing arm. I'm actually suspicious that the collar that the seal rides on were damaged first by a bearing failure which would lead to premature seal failure and then dirt and water damage. Inspect those bearings and seals regularly.