How to delete all mail in IPAD 1 and keep mail untouched in Gmail server.

I am pretty sure that your gmail account is IMAP. With IMAP email accounts, whatever you do on one device syncs across all devices, if you delete the email from the iPad it is deleted from the server. If you create a mailbox to store the emails thinking that will free up the space, that will not work either, because that mailbox containing the saved emails will also sync to the iPad.

The bottom line, is there is no way to do what you want to do.

You GMail account is set to IMAP, it needs to be set to POP3 then change your iPad to use POP3. It may require deleting your GMail account from the iPad then set it up again after changing the server settings.

https://support.google.com/mail/troubleshooter/1668960?hl=en

Most people prefer IMAP having their email synced between multiple devices.

It has been some time since I used POP3 on my iPad, it is just not a very good way to manage email between multiple devices. There is no check mark to simply change the Gmail account from IMAP to POP on the iPad. You will have to delete the account from the iPad removing all the email already stored on it. Once you have changed the server settings at Gmail you have to re-create the account on the iPad. Now that it is POP3 you should have a setting in the iPads settings to not delete the emails from the server when deleting them from the iPad. Once the new account is created on the iPad all the email on the server will download again and have to be deleted from the iPad. You will have to reset other devices as well, although Gmail may be able to handle both settings at the same time but they may conflict.

For your use, you may simply want to just use Gmail through Safari where you will be viewing your email directly from the server through the web browser. No messages are stored on your iPad, they are all managed on the server through your web browser. There is also a Gmail app available which may also leave all the messages on the server although I have not experience with the app.

When I had an email account set to POP3 I had the iPhone and iPad set to not delete messages from the server when I deleted them from the device. I had my home computer set to remove them from the server when they were deleted from there. It does work although each device is independent, reading a message on 1 device would not mark it as read on the other device. I ended up having to administer the email on 3 different devices even though they were the same account.

The problem you will soon have to bear is keeping track of what you are doing with each mail across all the platforms you may be using. When in POP, there will be no feedback on whether a message has been answered, forwarded, etc. And if you find that the IMAP index is burdensome, wait till POP unceremoniously dumps ALL the messages, lock, stock and barrel, onto the devices. The reason IMAP was developed was precisely to address POP's shortcomings.

If you find yourself overwhelmed by all the messages all the time in the current IMAP configuration, have you considered putting a time limit on how far back iOS' Mail goes in the timeline to construct the index? Note that this is a per-account setting. Settings / Mail, Contacts, Calendars / <your account> / Mail Days to Sync

Having just switched from Pop to IMAP, I still have the 'On My Computer' file listed just below my account in my Outlook Email Window. Those contain all my prior Pop Emails that I need to save. The new emails in IMAP that I need to save I simply won't delete on any device; I will make new folders on my On My Computer folder --which correspond to the same-named folders in my imap account--and a few times a year I'll drag over all those emails to the on my computer folders--which will copy them not delete them from my imap, then delete from my Imap as I need to. EVery year I start a new set of folders on the On My Computer folders to archive all the emails for that year. This backup --and there's probably a way to set your email up so it backs up automatically to the On My Computer folder--is a safety in case all your emails get deleted from your server by accident or server malfunction.