Would you say that Gatsby was a victim of circumstances or an instigator of tragic events?

This is ultimately a matter of personal opinion, but I believe that Gatsby was the instigator of his own tragic demise. Gatsby's motivations throughout the entirety of the book are hidden and dishonest. He claims to want nothing more than the love of Daisy, but his actions could be read as supporting an obsession with his own image. Gatsby becomes fixated on amassing a fortune in order to be "worthy" of Daisy's love, but it was quite clear that the lengths to which he went to do this were unnecessary, as he even engaged in illegal behavior. Additionally, Gatsby is an awfully demanding fellow for someone who believes himself to be offering up unconditional love. He tries to play an "all or nothing" game with Daisy and Tom when the news of their affair comes out, and he loses.
A more realistic man would have seen that Daisy—now a married woman who seems committed to that marriage, if only to retain the status and privileges that it offers her—is out of reach. Instead, Gatsby railed against the obvious and pursued her relentlessly, engineering opportunities for the pair to reunite. Even at the end, Gatsby can't seem to take a hint; when Daisy won't fully commit after their affair is revealed to Tom, Gatsby still attempts to sweep her away from the marriage he considers so disastrous. The fact of the matter is that if Daisy had truly wanted to leave Tom, she would have. She had an opening to do so and declined.
That being said, many people tend to view Gatsby as a tragic, romantic hero who simply runs out of luck and gets dealt a bad hand. So, my best advice is to read the book and decide for yourself!