There are three basic approaches to GP production which, unsurprisingly stem from how you get them.
A. Production and wonders. You can amass a lot of GPP/turn off of wonders and that can quickly farm a lot of GP. The downside here is that you lose fine control of the pool and things can be a mixed bag for the spec you want vs the spec you need. Mixing in NE may not be the best, e.g. abusing the synergy from Rep, B, and settled specs makes for a strong IW/Oxford city which the NE just doesn't match.
B. Food and specs. Pretty much any three food resource city will do for this type. Just get your slots, build the NE and go. Commonly people will run a lot of scientists in the early game for bulbing down the middle path. A less popular, but still viable option is to build the NE in a high food/shrine city (most often an AI cap you've taken), run mass merchants, and settle the merchants to keep your slider high and to have a nice WS location late game. Regardless a high food GPfarm is pretty flexible - you can mass any spec you can slot on command and you can easily get anything from caste. Dropping in the NP post bio or the globe at drama can be quite helpful (though the Globe may be better used in a dedicated draft city).
C. NP and forest preserves. You have to wait longer to get this one up, but it can be trivial to get 10 specs with this (tundra forest becomes useful). If you aren't snagging wonders this can make a very strong mid-late game GP farm if you spent the early game working cottages. Some of the best candidates for a NP/NE GP farm are astro islands that still have plenty of forest. The downside is you have to wait a long time before you can use this and when you do you may be lacking in slots, the upshot is that you can run a lot of specs without needing all the
and
infra.
What to accomplish with your GP, once you have a steady production?
1. Settle. As alluded
specs (GE, GPr, GSc) all get good synergy with Rep/B/Oxford/IW in the cap. With stone or marble this can take you a long way quick (mids is very helpful as is the ToA, AW, and HG).
2. Bulb. The bulb shots to philo (GSc), theo (GPr), edu (GSc), etc. are quick ways to get a tech lead you can trade. On immort or higher it becomes increasingly difficult to keep up in technology in the mid game. Mass cottages in theory give more than GSc, but the GSc gives you a monopoly tech now ... which comes with its own modifier when you can trade it to every AI.
3. GA. One of the keys to good GP farming is establishing a supply of diverse GP late game. Getting a 4th GA requires you get 4 differing GP and that can be hard to do if you are just sitting on wonders with NE. You can get some mileage from the free specs in the tech tree (Music, Econ, Physics, communism, and fusion), but there is value in a flexible GP pool, particularly if you can swing it without caste.
4. Corporations. If you game is going to last another 50 turns (ballpark) you likely can get a lot of mileage from founding corporations in your WS city and running those in FM.
5. Special uses. The oxford paired academy is pretty classic (though I think somewhat overrated). Using a GE to snag a free wonder (e.g. Verseille on a new land mass you have just begun to settle) can be pretty handy. Trade missions can buy for timely mass upgrades or let you deficit research at key times. On the flip side, these uses may be completely underwhelming (e.g. most culture bombs outside of dedicated culture shots).
Hope this helps.
in a single turn by bulbing Philosophy. How many cottages will do that for you? 
! I've just always felt sorry for those nice tiles - building farms on them instead of those juicy cottages... 
Any more advice is welcomed 
