Included in hadrian.contrib is hadrian.contrib.gravatar a gravatar template tag library.
Add hadrian.contrib.gravatar to installed apps.
Getting just the URL of the image:
<img src="{% get_gravatar email_address 75 %}" width="75">
Using the built in wrapper:
{% gravatar email_address 75 %}
Which produces:
<img src="{{ gravatar.url }}"
width="{{ gravatar.size }}" height="{{ gravatar.size }}" border="0" class="avatar" />