This group isn't about either Excel or gmail, but
a. when you Save As from Excel to a .csv format, the chances are that Excel
will
   - warn you that a multi-sheet Excel workbook can't be saved as a .csv .
In other words, you can only export the data from a single sheet - probably
what you expect.
   - warn you that some of the data may not export successfully into .csv
formt. That could be for a variety of reasons, but in essence Excel is
saying "Excel format is richer than .csv format". So again,that message may
not be telling you anything particularly significant in this case.
b. you should be able to open the .csv file in a plain text editor such as
Notepad. if you also have a description of what gmail expects to be in a
.csv file (I don't know), you should be able to work out from that what is
going wrong. It could be that your data has delimiter characters such as
commas, carriage returns, or double-quote characters in it.

Peter Jamieson