I can suggest 3 solutions:
1) Apply the AutoHeight method to every group row to automatically adjust its height to display the group row contents without clipping. Obviously, you need to do this in a handler of the AfterAutoGroupRowCreated event.
2) Use the AfterAutoGroupRowCreated event to change the contents of group rows when they are created.
3) Set iGrid's PrefixGroupValues property to False to avoid using the column header text in group rows.
I would also recommend that you read the Group Row Contents topic in the manual:

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It contains a good description of what you can do with group row values in iGrid.