Hi Benjamin

If you are redirecting your emails from your
main domain account in c-panel to your G-Mail account then
I do not think there is any way of not storing
your emails in your c-panel account.

Maybe you could set up a filter in your main account
to delete once redirected.

I find it is a good idea to have the mail saved
at my c-panel as a backup or you could just go into
c-panel each day and delete your mail.

Sorry I can not be more helpful but maybe someone
can be more specific