A good essay includes both!
Been a few years since I watched Gattaca but from what I recall there's a lot to pick from in terms of themes. A lot on morals and ethics, but also some existential-ish questions like 'who has the right to play god' and more personal, human aspects like the nature of love if you find that particularly interesting. Fate and such as well. It's a very excellent film. As for techniques, they should support what you have to say about the themes; 'the dark lighting and ominous score of [scene], when [character] says [something] supports the director's comdemnation of man's inherent desire to surpass his own limitations. The connotations of the aforementioned ...' etc is an imperfect but reasonable enough way to include both themes and techniques in your discussion.
As for developing your own interpretation, what does the film make you feel? Does it bother you that people aren't allowed to do what they want because they'd be more effective somewhere else, or do you agree? Pick a scene from the film that you enjoyed (if any
) and write down what made you enjoy it, you might come up with something that would suit an essay. Personally I like to pick a theme and then look for evidence that supports it rather than try and find a theme that fits the techniques.