perhaps if you think dualism it will help.
Jupiter is big, expansive, diurnal, masculine, hot & moist, active, generous, etc. (able to act & receive or alternating between the two while in it's dignity of honor, sag & pisces while exalted in cancer).
Fiery sag is warm & moist, masculine, diurnal, requires freedom of action & power, mutable
Gemini is mercurial, smallest, notcurnal, airy, wiry, mental, mutable also & requires freedom of movement in speech; masculine sign, yet mercury can be either depending on sign it's in & corporal conjunctions if any. mercury is hot & dry, but, like quicksilver, can change quickly depending on who(m) he/she's with.
So, jupiter has a hard time being is his big expansive and active self in a sign that is mental, quick and often changing, flitting from one thing to another.
In his fall of capricorn (which is opposite his exaltation in cancer) it is much worse.
(speaking from personal experience ~grin)
Saturn, ruler of capricorn is earthy, cardinal, hard, cold & dry, diurnal - but of grave manner (so unlike jovial jupiter), stern, discipline (jup hates that), takes the fun out of everything by being so serious and is the sun's only enemy, while jup is the sun's best friend/ally.
so, think of jup in gemini as the big beautiful centaur, teaching, playing music, teasing & playing with the little ones while watching over them (not well either mind you, but fondly, yet bored) half an eye peering at the guys doing archery and the other eye checking out the mares, wondering if he couldn't just split his time.......you get the picture.
while in sag, he'd manage all quite well and confidently;
in gemini, he'll wander off and not quite get back to where he should be. HOWEVER, if a benefic aspects him he receives help, if he has dignity in trip (and in gem he does) decan or term, perhaps placed in his house of joy (11) and not hindered by the malefics nor burnt up by the sun (cmb) nor cadent, etc, etc. He can achieve a little of what he promises, but in detriment he does need alot help.
There's alot to the art and science of astrology, eh?
sigh, alas, Jup in mine is cadent & fall AND conjunct saturn AND square mars/mercury, with itty, bitty reprieve (now & then) by trine to exalted waning moon (changeable & moody as she is). So, he promises much and delivers none (I believe he is what is called "accidently malefic").