You will be able to do it simply sending a mail to your email, appointments information is send as a ics file.
in gmail when a ics file is revised it reads the data in the ics file and add event to the google calendar.
so you will be able to get the job done simply by an email, you will have to read about ics file and get the syntax of it.
sample ics file looks like
BEGIN:VCALENDAR
VERSION:2.0
BEGIN:VEVENT
STATUS:TENTATIVE
DTSTART:"+NewStartDate+"T"+NewStartTime+"Z
DTEND:"+NewEndDate+"T"+NewEndTime+"Z
LOCATION;ENCODING=QUOTED-PRINTABLE:"+Location+"
CATEGORIES:Profissional
DESCRIPTION;ENCODING=QUOTED-PRINTABLE:"+Description+"
SUMMARY;ENCODING=QUOTED-PRINTABLE:"+Subject+"
PRIORITY:3
END:VEVENT
END:VCALENDAR
I found this when i google, hope this will help https://gist.github.com/jakebellacera/635416