This quote is important because it not only establishes the essence of the Gatsby character but it also foreshadows the very nature of the story and its primary themes: idealism, aspiration, and loss.
Nick sees Gatsby here as a figure untouched by doubt, convinced that if he simply maintains a pursuit, a particular posture, or develops a dedicated strategy, he will be able to achieve anything he dreams of.
By comparing Gatsby to a machine, Nick implies that there is a purpose to the "unbroken series of successful gestures". Like any machine, there is an aim to the work, a point to the gestures; a defined goal that is being approached. In this case, the goal is not entirely clear, but the means by which this goal is to be attained is related to personality.
The machine of Gatsby's personality is calibrated to respond to social cues in such a way as to create an idea of elegance and importance. He is adept at the presentation of being receptive to success, to achievement, and to status.
The implication here is that Gatsby has carefully and successfully constructed a personality that is in pursuit of an aim. Gatsby is not the way he is due to a response to events or due to impressions taken from the cultural world around him. He does not participate in "flabby impressionability".
Though his personality might be best seen as a persona (a mask; a facade), and it is therefore a "creation", this persona is not a show for art's sake. It is not a performance but a means to an end.
Gatsby is capable of being (to Nick) obviously "made up" or "put on" yet still holds on to the sure conviction that his gestures will lead him to his target. This impresses Nick as being extremely hopeful.
We might draw an analogy comparing Gatsby to an actor here, where we see Nick characterizing Gatsby as an actor who believes that if he plays his part without deviation that part will cease to be acting and will become reality.
Gatsby believes in his own potential to such a degree that Nick sees the belief as "romantic", as if Gatsby's life were, to Gatsby, a poetic tale wherein he is the inevitable hero.