How can I connect to gmail account using Mail on iPad

Given the age of your iPad, you will struggle to find any Apps that remain compatible with your long-ago obsolete iPad - and will similarly be unable to update iOS to a current version.

Not sure which iPad you have? Identify your iPad model.

Here is a list of the highest version of iOS supported by the older models of iPad (for simplicity - “generations” are shortened to a number) that are restricted in their ability to be updated:

iPad1 - iOS 5.1.1

iPad2, iPad3 and iPad mini1 - iOS 9.3.5 (WiFi Only models), iOS 9.3.6 (WiFi & Cellular)

iPad4 - iOS 10.3.3 (WiFi Only), iOS 10.3.4 (WiFi & Cellular)

iPad Air1, iPad mini2 and iPad mini3 - iOS 12.5.5

NB: iOS/iPadOS 15.5 is current.

You still be able to use built-in Apps, such as Mail - to which you will need to add your Gmail account:

Add an email account to your iPhone, iPad, or iPod touch - Apple Support

You may need to create an App-specific Password to access your Gmail. Google have changed their authentication requirements for Google Mail mail accounts configured for password authentication - and for non-Google mail clients, now requires use of an App Specific password for each mail client/device. 

You will need to generate a unique password for your iPad Gmail account - that will differ from your normal Google Account Password. AOL, Yahoo, Google, Microsoft now all use App Specific Passwords when using anything other than their own email client Apps.

If you need to generate an App Specific Password, you’ll need to log-in to your Google Account management webpage - using a web browser - and generate the Password from the appropriate account security page.

More information can be found here:

https://support.google.com/mail/answer/185833?hl=en-GB