Your problem has nothing to do with jqGrid and everything to do with css.
First things first… There is no property as float: center. It's either, left, right or none.
Next… you did well to wrap the grid in a div. The first thing to do is float your vertical menu left, then you can set the wrppaer div's margin: 0 auto. This will place the wrppaer div in the middle of the available space.
Hope this helps. Maybe a more experienced CSS'er can be of better assistance.
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