[akots]

You are obviously doing something wrong. I'm not sure what the problem is but IMO it might be that you start GP-based economy too early and run it all the way through whereas it is not a good idea to base your whole game on it. It might be that you might need to:

1) build more cities (at least 6-8 as early as possible if the economy can handle that at 30-40% research).
2) If you cannot, build yourself more units and go conquer these cities from neighbors.

There is always use of some terrain unless it is all riverless plains without bonus resources. Grasslands can be farmed to work hills or build up great people or cottaged if there are no rivers or hills nearby. Plains can be workshopped or cottaged or farmed in a worst case scenario. Flood plains can be farmed to build up great people or to work nearby hills or they can be cottaged as well.

Great people farming usually is better in caste system with pacifism and national epic. You can then farm 3-4 great scientists in no time in the middle of the game to accelerate your way towards Liberalism or towards Astronomy depending on the map. Great people farming very early in the game is not too useful except may be first great scientist for Academy in the capital or on higher difficulty levels for lightbulbing philosophy to trade yourself even in techs with the AI. Otherwise, it greatly slows down growth. So, early in the game, slavery is usually preferred as you can whip improvements or settlers/workers or units.

Farming for GE makes sense only with philosophical trait and farming for GM makes sense only if you run universal suffrage (built or conquered Pyramids) or need massive instant upgrades. Farming for GP is usually not required. Farming for GA is needed only if going for cultural victory. Otherwise, GS are always better because they give better techs and more beakers towards these techs.