The heavens are divided into 12 zodiacs and each zodiac into three sections. The sun, the moon, and the nine planets must be located in these 36 options.
First we know the sun was in Aquarius (from GK1).
For the Aquarius dates of January 20th to February 18th, six planets also remain constant. Mercury remains in the house of Capricorn, Jupiter in the house of Pisces, Saturn in Aquarius, Uranus in Virgo, Neptune in Scorpio, and Pluto in Virgo.

I ruled out 1964 at this point because of the triple conjunction of the sun, Saturn, and Mars. The interpretations made no sense in any of the three combinations.
The moon, during this same period, can be in any of the twelve houses. In looking at the interpretations for all twelve I found this decision easy. In 11 of the 12 houses the moon and sun relationship yielded interpretations that didn't fit Gabe, but if the moon was under Pieces the interpretation was very different from the rest, and fit him well.
The moon was in Pisces on two days: the 27th and 28th of January in 1963. The only other variation of the planets for those two days was the moon's conjunction with Jupiter on the 28th. The conjunction made perfect sense and is how I decided on January 28th 1963.

Here is Gabriel Knight's horoscope based on this date:

With the Sun in Aquarius and the Moon in Pisces.

"You are intuitive, something of a mystic, leaning on your hunches and your inspirations and capable of going far, either toward truth or toward fallacies through the following of your swift mental processes.

You have a feeling for poetry that makes it appear a science, and for science that makes of it a sort of poetry. Your combination of the real and the abstract, the imaginative and the demonstrable, the mystical and the earthly, gives you a broad hold on all branches of human knowledge and thought.

You need a good education, strict discipline in logic, and the development of a respect for accuracy and fact. Without this intellectual training you can become a first-class visionary, spiritualist, medium or clairvoyant, believing in ghosts, fairies, and hobgoblins. In any case you will be fascinated throughout life by the unseen, the mysterious, and the unusual, which may give you strange fears and phobias and cause you to be gullible on some points at the same time that you are completely rational on others.

You lead a private life of your own, into which few other people ever penetrate. You may have delusions either of grandeur or persecution, genius, a belief in witches and warlocks, a propensity to believe in alchemy, or that there will befall woe if a black cat crosses your path.

There is an odd admixture of the irrational with all the otherwise completely sane things that you do and believe but so great is your grip on reality that these oddities rarely amount to anything and really lend colour and mystery to your personality. In your emotional life you are idealistic, loyal, sensitive, and easily imposed upon and will be hurt rather than angry at being fooled."

With the moon in conjunction with Jupiter:

"Ethics are your god. You believe in your own personal worth from a moral and ethical viewpoint. Your abilities may be high or low, depending on other things, but you are utterly dependable and can be counted on to do the right thing and think the right thoughts. This doesn't make you puritanical or cold.

You have a naturally cheerful and optimistic nature, and are very sympathetic toward other people and their troubles. You have a genuine feeling for your fellow man, his problems and difficulties. Your occupation is more than a means of livelihood -it's a conviction and a passion. You have a certain nobility of approach and of motive - you couldn't be mean or petty if you tried, and thus your popularity and luck are well deserved, and nobody grudges them to you. You can be a stern judge but you will always temper justice with mercy. You will condemn people, perhaps, morally, but be reluctant to punish them.

You are virtually assured of a long and honorable career, of great personal influence and public recognition. Because this conjunction happens in Pieces, you're well placed financially, and very charitable in a tangible fashion. The humanitarian motive is uppermost and makes you an extraordinarily fine and well balanced person."

The Aspect of the Moon and Neptune:

Charm and grace of manner and speech mark you - a certain fluency to your movements and manner of expression attract attention. You are rather striking personality, in a quiet and unassuming way. You have a keen sense of other people's feelings and wishes.

You put two and two together instinctively and thus are able to draw conclusions that are lost on less intuitive souls. Beauty in all forms, has a strange effect on you, and conversely discord, strife, and loud noises throw you off. You will give anything for this peace and quiet, and for this reason frequently go off by yourself for periods of repose and relaxation, far from the madding crowd.

If there are any artistic or creative indications in the horoscope, this will assist them greatly, although by itself it gives the appreciative, receptive artistic qualities. You have a love of music and really require it, finding comfort and relaxation in the rhythm, tone, and colour of what you hear.

Your sense perceptions are acute and sights, sounds, odours, colours, and the feel of things give you a curious sensuous satisfaction. You should cultivate objectivity, and detachment from you private aesthetic world, which pleasant though it is can interfere with realistic problems."

The Aspect of Moon and Pluto:

"The important lessons in your life will be learned through your relationships. Regardless of your sex, the mother-figure will shape your reactions to all women. You base your conception of femininity and nurturing on the feedback you received from your mother.

You tend to hold on to the past and act on your early conditioning until you realize how your unconscious reactions and motivations hold you back. You may feel you need freedom, thus avoiding close emotional relationships, when what you really need is to take a direct and personal responsibility for the direction of your life and the satisfaction of your emotional needs.

Compromise is one of the key lessons of your life."