If your not using the webpage (www.gmail.com) to do it but instead on a client software (email program) then either contact Apple and their Genius can walk you through (the reason for the high cost of Apples is for their great customer experience) or just use www.google.com and search for Gmail on Safari Browser or whatever 'email' software name your using.
Normally no one is using email clients (Outlook, etc.) these days and instead just go to the webpage itself (www.gmail.com) and click the button to 'send' a email or 'reply' and pops open a webpage, which you input in your email, click send and off it goes. If there is a problem ON Gmail.com website you need to call Google for support.