ArgumentError (SMTP-AUTH requested but missing user name) - ruby-on-rails

Question

I am working on a basic app called PhotoApp, where I am learning how to set up SMTP with rails. I have used basic Ruby with SMTP sometime ago, and I know my email works, just the emails sent gets to spam folder.
But when I am trying to configure SMTP on Rails, I am getting ArgumentError (SMTP-AUTH requested but missing user name):, and that's the primary reason why the app, when pushed to heroku, crashed every time a user signs up.
Here's my config/environments/development.rb:
config.action_mailer.default_url_options = { host: 'localhost:8080' }
config.action_mailer.delivery_method = :smtp
config.action_mailer.smtp_settings = {
address: 'smtp.gmail.com',
port: 587,
domain: 'example.net',
authentication: 'plain',
enable_starttls_auto: true,
user_name: 'gmail_email',
password: '...'
}
config.action_mailer.raise_delivery_errors = true
config.action_mailer.perform_deliveries = true
I am running the server on localhost:8080, in development mode, so my host is set to localhost:8080.
Apart from that I am not able to get my app working with my gmail username with SMTP in rails.
Edit, this ruby code works:
#!/usr/bin/ruby -w
require 'net/smtp'
message = <<~EOF
From: mygmail#gmail.com
To: someone#protonmail.com
Subject: Hello!
Hello!
#{"HELLO ".*(50).delete_suffix(?\s)}
EOF
Net::SMTP.new('smtp.gmail.com', 587).tap(&:enable_starttls_auto).start('gmail.com', 'mygmail#gmail.com', 'mygmailpass', :plain) do |smtp|
smtp.send_message message, 'mygmail#gmail.com', 'someone#protonmail.com'
end
But as a newbie to rails, I can't get actionmailer working when the same email is used in the config file...