On the gmail ap, my inbox is always empty... Even when I hit refresh.
When I look in the "All Mail" view, I can see new mail marked with the blue inbox but they still dont appear in the inbox page itself.
Try clicking on the gmail mailbox--then hit menu--- more--settings--then send & recieve. Look at your download options..i had mine set at one day and was not getting anymail so I set it to 3 days and all my mail appeared!!
Hope this helps