Hi Kpseshu,
Thanks for coming to Apple Support Communities for help today sending Gmail on your iPhone. It sounds like you have done some great troubleshooting already.
Have you seen: If you can’t send email on your iPhone, iPad, or iPod touch. Typically removing and adding the account will resolve an issue like this. Did you follow:
"Remove your email account and set it up again
- On your computer, sign in to your email provider’s website. Make sure that all of your email is there or make sure that your email is saved somewhere other than your iOS device.
- On your iOS device, go to Settings > Mail, then tap Accounts.
- Tap the email account that you want to remove.
- Tap Delete Account.
- Add your account again."
Have you tested sending Gmail elsewhere like the webmail portal?
If you can send mail elsewhere, try removing the account again and also delete the Mail app: Remove apps from iPhone. Restart your iPhone, redownload Mail from the App Store and install Gmail again.
Let us know how that goes.
Have a fantastic day!