I would say because it's .NET, and decompilation involves downloading ILSpy and spending about 30 seconds of reading your nearly raw source (including variable names)... I probably wouldn't even bother, being as anything outside of sending all requests to a server YOU own that will do the auth, anyone that isn't terminally lazy can decrypt the password no matter how much time you waste on heavy encryption on locally stored files.
I'd probably make it so that you can change the SMTP server, and make sure if the software gets redistributed that they hand out the one with the blank SMTP server settings, and basically write off the GMail account if they don't.
If you want to feel a tiny bit more secure, you can drop in something simple, like an AES key + the encrypted password, and use that as the config file, but that isn't going to stop any competent programmers, but at least you didn't spend days or weeks trying to develop something over-complicated that I can still crack in the same amount of time (being as I just have to add your .exe as a resource, execute the call to pull your config, and read the raw variables in memory).
Though you might as well just base64 it so it isn't raw (because this is stupid-easy), because as I said, anything you do is fairly easy to crack.