You should not ask for a users gmail username and password; there is a way to generate an authentication key for your app that allows the user to revoke the access later as well. I do the same for my mail delivery system.
It's quite easy; here is my C# console app that generates the creds and uses it to send an e-mail. https://imgur.com/a/ZjZUFqS
It's quite easy; here is my C# console app that generates the creds and uses it to send an e-mail. https://imgur.com/a/ZjZUFqS