Do you mean the pivot point is off center? Gizmo is the name for the center of a modifier and is different from the center of an object.

You probably should have the pivot point of your wheels properly centered as I think the software uses that axis to determine how to rotate the object.

To re-adjust the pivot point
1 - select the object
2-  click on the heirarchy tab - (looks like a little TO chart)
3 - click on "Affect pivot point only"
4- you can now either select one of the other options like center to object, or use the move tool to get the pivot point exactly where you want it.
5- when finished - unselect the Affect pivot point button and you should be good to go.

Remember - the object will show the pivot point in different locations, depending apon which center mode is selected, up in the top menu. You'll want to have the pivot point center mode selected. That's the one where the two different size boxes have a red dot at the bottom of each box.