how to reinstall deleted emails from a gmail account

What interface are you using? Mail? Gmail's own app?

If Mail, go back to the top level of Mail and you will see a list of folders each mail service has set up. Find Gmail's trash and then tap the item and move it out of there and put it in the desired location.

I don't use Gmail's app and you would need to as on a Gmail forum about that.

It is not clear from your post on how did you "accidentally" deleted all emails.

Did you remove the Gmail account from the iOS mail app?

Did you remove them from the Gmail App?

Did you check for the email on a web browser?

Remove your email account and set it up again

Make sure that all of your emails are there or make sure that your email is saved somewhere other than your device. On your device, go to Settings > Mail, then tap Accounts. Tap the email account that you want to remove. Tap Delete Account.

If you can't receive email on your iPhone, iPad, or iPod touch ...