Most of the times I try to stand my ground and be more defensive against the person, giving them a stare or trying to defend myself. There are times when I fantasize a great deal about the person being tortured, at times even months after the offense has happened and then do some research on the person to see who I am really getting into it with.
Is the person trained? Is the person socially well connected?
I always avoid them and blatantly ignore them if I run into them again unless they apologize, then I slowly start to forgive but I never really forget. I do ice people out that I feel have treated me poorly, often blocking them on all online networks that I can and if possible, gossiping about them as soon as the opportunity presents itself.
Mars in Gemini fights verbally. If you gossip about people you feel have treated you poorly, that's a Gemini way of fighting. If you ice people out and refuse to speak to them, that's also Gemini: you're punishing them by withdrawing your words.
Your Mars is also sextile your Mercury, if you use a wide orb. It has some help from the ruler of its sign. Mercury is not only words, it's also thoughts and perceptions and details. If you're paying attention to details and making plans (whether you carry them out or not), and thinking about revenge and posting about it in forums... that's also quite Gemini and Mercurial.
To be honest with you though, I have avoided fights most of my life and have not really been pushed to that limit which is why I want to know what my worst side is capable of.
Ideally, I'd like to be the guy who can ruin a wicked person's life in a creative way but within legal means. By this I mean getting some dirt on them and then exposing it to the public until the reputation and image is in the garbage can or coming up with some sort of scheme where they are getting their just desserts. I hate it when scumbags get ahead and don't pay the price for the evil they have committed.
A lot of time is spent thinking how I can teach this person a lesson without getting myself into legal trouble, what kind of a scheme I can come up with where justice is served.
If you can imagine doing it, you're capable of it. But capable of it doesn't necessarily mean you'll do it. Maybe you'll decide it's not such a good idea after all, once you've considered all the pertinent details. Maybe getting the kind of dirt on someone that would completely ruin their reputation turns out not to be realistic--or not realistic for you to spread without serious consequences for yourself. Maybe, depending on how strong a sense of conscience you have, you would feel so guilty if you actually did what you contemplated that it's not worth it. That wouldn't mean you're not capable of doing it, just that you have stronger reasons not to.
I think for those of us with Gemini Mars (at lease some Gemini Mars folks) thinking about and talking about and imagining revenge on people who've made us angry is often the way we work out anger. If that's so for you, then it may not ever be necessary for you to actually carry out any of your revenge schemes--because the emotional need they would meet for you if you carried them out is fulfilled by just coming up with them (and not carrying them out also means you don't have any unintended consequences to deal with). Coming up with the revenge scheme is, in and of itself, the process that relieves the hurt.
And if you do... well, I'm Gemini Mars myself. I once (when I was young, just a little younger than you are now) played a quite nasty prank on a friend who had deliberately hurt my feelings. He figured it out before anyone could really get hurt (fortunately), and then... he came over to apologize for the nasty things he'd said to me. It had sunk in for him just how angry I was. Once he apologized, though, I wasn't angry at all anymore.
But I don't think that Gemini Mars is all that's driving you in matters of revenge. That you hold on to anger for so long, seems very Plutonian to me. Mars in Gemini, all by itself, woudn't do that. Gemini is an impulsive sign: feelings processed through Gemini come and go easily. But Scorpio/Pluto feels deeply, holds onto things, and doesn't let go. And won't be satisfied by an act of revenge, even if you think it would.