How long ago was that? I have some OM gcs that I got last year and the cashier said no go on buying gcs with om gcs. Wouldn't even try it........... said the register wouldn't take them. This was at a store that gave me no hassles in using rewards to buy gcs when it was allowed.
This was end of October of this year that I tried it.
Worst case scenario wouldn't you be able to purchase an item and return for store credit and use that to get the GC if you wanted?