Gestalt therapy with adolescents is possible, but great attention must be paid to the issue of organization. Adolescents tends to be rather variably organized and must in part learn to put their thoughts and perceptions in order. I think that in general the more heavily expressive techniques (e.g., psychodrama type approaches) should be used with great caution with adolescents. They're usually dramatic enough without the therapist adding to it!
With a child with ADHD, I would tend to focus on teaching awareness techniques... how to shuttle between external and internal awareness, for example. How to create focus points. How to segment the field into manageable pieces (e.g., covering most of the page of math problems and only uncovering them one at a time). It seems to me that a key issue for people with ADHD is that they are not able to organize their perceptual fields and their thinking and attention effectively. I'd focus on that initially, and then look at whether the client can develop some understanding about the pattern of his/her attention. What specific stimuli tend to be distracting? Is there a temporal pattern to their distraction that can be worked around?
Another crucial issus in Gestalt therapy is that of self-support. How does the individual support him/herself, and how does his/her self-support style affect his/her attention?