I am trying to add my gmail account but can't do so. Can anyone help, please?

Have you tried but failed? What hasn't worked for you?

There are two ways you can add a Google account (or any other).

1. System Preferences > Internet Accounts > Click on the + button at the bottom left of the panel > Choose your service > Click on Open Browser > Enter Your Credentials and follow the prompts. This will give you options to install more than your email service - for example Google calendars.
2. Go to Mail > Preferences > Accounts > + button > Choose your email service, enter your credentials and follow the prompts.

I just added an old and almost forgotten gmail address to my Mac's Mail via the System Preferences route (method 1 above). After a pause it added the account to Mail, then another pause followed as it downloaded the four very old emails in the In box. I suspect that if you have heaps of mail it will take a while to cogitate and download them.