from django import template
from django.contrib.sites.models import Site
from django.utils.http import urlquote_plus

register = template.Library()

@register.filter
def google_calendarize(event):
    st = event.start
    en = event.end and event.end or event.start
    tfmt = '%Y%m%dT000000'

    dates = '%s%s%s' % (st.strftime(tfmt), '%2F', en.strftime(tfmt))
    name = urlquote_plus(event.name)

    s = ('http://www.google.com/calendar/event?action=TEMPLATE&' +
         'text=' + name + '&' +
         'dates=' + dates + '&' +
         'sprop=website:' + urlquote_plus(Site.objects.get_current().domain))

    if event.location:
        s = s + '&location=' + urlquote_plus(event.location)

    return s + '&trp=false'

google_calendarize.safe = True



And this code can be invoked via:
{% load project_events_tags %}
...
<a href="{{ event|google_calendarize }}">+ Add to Google Calendar</a>