The Myers-Briggs Test is one of the most trusted personality evaluations in the world. Going far beyond the internet quizzes we all know and love, if administered properly the results have a high chance of being shockingly true to life. The theory behind this assessment makes a point to say that, statistically, the happiest couples tend to have two opposite values (such as introversion and extroversion, combined with a 'thinkers' outlook and a 'feelers'), and two conducive, similar ones (such as intuitive actors, and those who choose to view the world through a more perceptive, straight-forward, cut-away-the-crap filter).
Surprise, surprise -- Gippal and Paine fit this theory perfectly. Not quite opposites attract, nor carbon copies, their viewpoint on life is shockingly similar; it is merely the way they express it that occasionally differs. Perfectly epitomizing the yin-and-yang, they are different personalities, yet each has enough of the other to fit together perfectly. This is essentially what makes Gippaine such a plausible, interesting couple.
Gippal is an open, forthright person. He has no hesitation in telling Nooj that 'when a Deathseeker dies, that's no apology'. Likewise, Paine has been known to bluntly state the truth as she sees it. There would be no secrets and silly games between the two; like the mature adults that they are, they could easily sit down and talk about their problems with no egotistical pricks to their veneers. They would understand that the other is merely expressing their view. Gippal wouldn't back down from her if the cause was right, a trait I'm sure Paine greatly respects, yet he would also let a stupid, immature issue die quickly.
Both are extremely independent -- each tells Yuna at one point that they do not need her help with something, and that they can stand on their own. However, when the time comes, both are willing to ask for assistance, and both do it without blaming the person they turn to as many prideful people often do.
Gippal obviously respects Paine immensely. He is comfortable enough in himself and his own accomplishments that he does not feel the need to ever deny someone the admiration they deserve. This would appeal to Paine, who is also confident enough to give respect where respect is due (or, in Rikku's case, she giveth then taketh away ;D). They could lift the other up in times of trial, and provide a needed, similar support system. Also, people with the confidence levels of Gippal and Paine typically are attracted to those of equal self-assurance. They do not want someone who is too dependent or clingy, and it is almost assured that they would never be this way with each other. They understand the need for space, and easily give it.
Gippal is often recognized for his relaxed, take-life-as-it-comes attitude. Paine is often criticized (or, in my case, respected) for her sarcastic, yet focused, remarks. While most people seem terribly offended by Paine's blunt nature, Gippal never blinked an eye. He easily takes her words, perhaps discards them, perhaps ponders them, then moves on and keeps smiling. He is extremely resilient, and openly forgiving and understanding. Furthermore, he never appears to give up; no matter how many times she iced him, chances are he would probably still come back with open arms, a smile, and a witty comment, much as he did with Nooj and Baralai. Even at Paine's coldest, he would be able to get through the frigid exterior with his warmth, and stand strongly and for always beside her.
Both Gippal and Paine make a point to remain non-political. Divisions and alliances are not as important to them as taking a spin in an airship with the wind through their hair. The little things in life -- travel, laughter, adventure -- appeal to both, versus large-scale movements. Even the Machine Faction has the distinctive air of a group of kids getting together on a Saturday to tinker with some machines. Gippal most likely intentionally keeps it this way, which is extremely conducive to his personality.
The two were both left relatively untouched by Shuyin. Though the pyreflies did invade them, they were not harmed or inhabited by Shuyin himself as Nooj and Baralai were. Having gone through the pain of seeing their dear friends possessed (and violent toward them), they would need someone else with a similar memory of the events to turn to in order to talk it out.
In the Crimson Spheres, Paine appears to find Gippal amusing. Not one to quickly laugh, she openly chuckles, and even lets him use her recorder. Remember, this is Paine we're talking about: I'm sure people have lost vital organs for less.
Their banter would be extremely amusing. Adept at wordplay, the two could undoubtedly go hours. When Gippal got a little too full of himself, she could zing him right back down; when she finds herself too stern or serious, he would easily be able to loosen up her mood, and invite her to live a little bit more.
Gippal is a good deal more focused than people tend to give him credit for; he would not take easily to constant hectic, bouncy, manic activity with no end, as he himself is energetic, but not hyperactive. Paine would do badly with continual negativity and seriousness, as she would find herself bogged down and depressed as an end result. As stated before, the key is to find a partner to temper these aspects of yourself, thus creating a 'yin and yang'. Such a balance is highly encouraged in most every action of life, and this would be no exception. While opening each other to new events and ideas (Gippal might like a weekend in Luca to see a blitzball match, Paine might have them take the next one in the serenity of seaside Besaid), they would also be extremely flexible with the give and take. Neither are selfish, and both often work very hard to avoid conflict. I can see them open to negotiation and occasional small sacrifices.
While masculine, Gippal would never try to dominate Paine, or expect her to conform to the stereotypical housewife role. Paine would be free (and encouraged!) to be herself, and never have to worry about being molded into something she is not.
In the end, it is the balance and equality of Gippal x Paine that appeals to me the most. Neither is above the other, neither is more powerful, and each could check the other, while enhancing their own personalities and viewpoints. Gippal would not weigh upon her with clinginess, and she would not fangirl over him in a stereotypical, mindless way. Something tells me Gippal has gotten quite enough of that over the years, and would be more interested in someone who was willing to challenge him. Easily bored, Gippal and Paine would both definitely, above all, need the stimulation.
Most importantly, it is an extremely pretty couple. :D