65gpt is not unusual at 100BC, for a golden age, and is achievable even without a GA if the AI in question conquered another empire in the earlier eras. I guess the real question is why you DON'T have that kind of income? If the AI has twice your score and a much higher income than you, on Prince, it means that YOU have done something wrong. It's not the AI cheating, it's you just not playing optimally.
Sometimes it's a question of luxury distribution; you might only have one or two luxuries, while Hiawatha's starting location might have had more types. If this happens you'll almost always have multiples of one luxury, so you should be trading your spares away for whatever you can get.
Sometimes it's a question of city-states. Bribing a city-state to gain a new luxury is just not as cost-efficient as just building a Colosseum/Theater/Stadium for yourself, although there are obviously other benefits (food/culture/units). So you have to know when it's worthwhile to make a CS ally and when it's better not to.
Sometimes it's a question of terrain. If he has El Dorado in his territory then that's +10 gold per turn that he's been getting since the start. If he's in grassland and plains while you're futzing around in tundra or desert, it gets worse. Likewise, if he's got plenty of rivers or improved forests (and as the Iroquois he should have plenty of forests) and you don't, it's a big imbalance.
But usually it's just a question of number of cities. Early expansion or conquering pays off in the long term; if you're still limiting yourself to the same four or five cities you had in 2000 BC, then you'll fall far behind. The fact that Hiawatha has a much higher score probably means he's picked up more of the Wonders, and a few of those can have huge impacts on the ability to expand. (Forbidden Palace, for instance.) If he's picking up these Wonders then he probably has a significant tech advantage on you as well, and at certain points in the game (whenever you unlock Banks, Factories, or Hospitals, for instance, or key offensive units like Tanks or Bombers) this can translate to a huge discrepancy in power.
The bottom line is, if you can't easily win a game on Prince, you have real problems, because that's the difficulty where the AI gets no cheats and it's only a question of whether you, a human, can outsmart an AI that's frankly underwhelming most of the time. You'll learn more tricks as time goes on, of course, but the real "break-even" difficulty is probably King, where the AI's bonuses offset its inherent stupidity.
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