GMail cannot open .xlsx, and I don't think it's ever been able to.
However, on a stock Note 3, you can open .xlsx from within gmail and via other methods - using Polaris Office. By default, gmail will use that to open .xlsx files on the Note 3. Note that when you open from gmail, and then hold home to get the task manager, it shows that window as being gmail, not polaris.
My guess is that you removed or froze Polaris Office. Put it back or defrost it, and see if it works.