According to your log files, something else is already listening on port 25.
from stunnel.conf:
; GMail ssmtp settings
[ssmtp]
accept = 25
and from error log:
That means if you're running Window Server, you should use a different port than 25, as something else on your system is already using that (for SMTP, presumably). Have you simply tried a different port, possibly higher than 1024 (like, 1465), then configure the client to connect to localhost to that port (1465). Then it should connect to your gmail SMTP server. If you can turn off SMTP on your Windows 2012 server, I would try that first.
I don't think this is a stunnel issue but an issue in trying to create an additional service listening on a port for which there is already a listening service.
Regards,
-Rob