100gpt would be a massive income in the early game - there's been times when I've shut down research totally (with the Great Library picking up the slack) and STILL not got 100gpt!
Keys to generating a large income and research at a decent rate:
1. Roads and rivers: if you don't generate commerce from your tiles, you'll have neither research nor income. Road every tile you work. Work as many river-side tiles as you can.
2. Go to sea, young man! Coastal towns are bad for shields, but generate lots of commerce from coast tiles.
3. Lots of people. Get your population up, and get them working the commerce rich tiles. The more people you have, the more money will be generated.
4. Multiply that commerce. Libraries and Universities (and Labs, later) will boost your research, allowing you to set a lower %age for the same tech pace. Similarly Marketplaces and banks. Plus market places will keep people happier (with luxuries) allowing your entertainment budget to be lower.
5. Trade, trade, trade. Sell those techs you discover to all the AIs; squeeze them till the pips squeak. Do it right and the AI will be paying your maintenance costs, while you spend all your income on the next tech. Repeat until 2050AD or liftoff, at your choice.
6. Trade, trade, tarde (2): and those extra luxuries you have - turn them into $$$. two of the same does nothing for you, convert it to income by selling it to an AI.
A nice side effect of trading like a maniac is that it'll make everyone like you - reducing the risk of wars.
7. A free people is a prosperous people. Get into one of the 'nice' governments as fast as you can. Republic and Democracy have decreased corruption and increased commerce; both help to increase your monetary base.
Personally, I don't target a gpt income; I either go all out for tech pace, and only care if that means running a prolonged deficit, or go for zero research, and get as much money as I can. I'm not sure a middle ground approach is all that efficient.