Military intervention against a GP is generally counter-productive (causing you a lot of dead workers and soldiers), unless you've got a C-B to give you something meaningful and a lot of allies to assist, and still doesn't give you much of a score increase. You're better off pursuing an economic or cultural advance, and gobbling up a couple of non-civs to provide the additional resources and manpower to boost you into the big leagues.
Generally, a long-term strategy of increasing Literacy, followed up by researching techs leading to higher educational impact, Plurality, and Prestige, will pay off down the road. If you can make it to #8 spot, then you can work on clawing out a Sphere of Influence to insure a steady supply of raw materials that you can actually buy, as well as to provide a market for your own goods.
After that, you'll want to prioritize the techs to get Machineguns and the chemistry advance (can't think of the name) by the early 1880s, to lower your minimum life rating so you can colonize. By that point, you'll want to have previously grabbed a colonial concession from some uncivilized nation along the coast of Africa or in the Far East, integrated it as a state, and built a port. That will allow you the range to colonize those areas when the big rush happens.
Then you just have to defend it.....