It is very strange to say the least. I am at a loss to fathom out that logging-in to Gmail would bring up onto your screen anything involving your bank; unless at some time you have been e/mailing your bank.
I Do Not use Internet banking so find it difficult to advise you a course of action. The best I can think is along the lines of what has been said in effect by Mosaic....Contact your Bank Immediately and change your password.
Also, in case some nasty item has entered your computer, give a quick scan with Malwarebytes. Download and run the Free Edition from :-
http://www.malwarebytes.org/free/
Do Not accept the 14 day Trial of the Premium edition; it's Not necessary.
Hans.