I think the best I can offer you from so far away and with such a short time for your presentation would be
(a) Go to the aricle you'll find at the top of this page in "About Gestalt Therapy" and you'll see one of the best descriptions of Gestalt therapy I've ever found in one article... it also includes a number of little vingettes or case examples. This should be a good base to start with for theory and examples... then
(b) have a look at what some current practitioners are writing about on this web page... particlarly the articles on mental illness, drug depedence and dual diagnosis.
I'd then consider cutting out the defining principles of Gestalt Therapy found in Yontef's article (at the begining) and handing them out to the students in the class. I'd then also hand out the little vingettes and some of the writing on current practice here on this page and I'd ask the students to see if they can see these priciples in operation in the case examples. You could also ask them to consider the application of these principles to their own work and any similarities they find with other approaches, such as CBT.
Hope this helps