In other words: you enter to Gmail webpage, click on "Write" or
"New" and write a new email. Or click on "Reply" and write your
answer to a email received.
All of this, you are doing it inside the Google Mail webpage. Is not a
program.
If you want use Gmail as a predeterminate email program, you have to set
up an email client. In example: Thunderbird. It is 5 seconds. You
enter your Gmail data (user and password) and Thunderbird does the
rest.
I hope you can understand what I wrote because I'm a Spanish speaker,
and sometimes I don't know how to express my ideas in English very well.
Regards,
Lailah