That would be the prospect of 6 farmed flood plains. I don't know what else you'd call a pop 7 city with the potential for 31 food and 8 specialists? 'Tailor made GPP farm' is not an exaggeration nor an expression of enthusiasm.
Actually, that doesn't make a very phenomenal GPP farm. Its principal problem is that it requires too many pop points to work food tiles to get large and generate GPP quickly. A city with 2 fish and a corn/wheat is singificantly better because it can get to the appropriate size very early and then run x specialists and 3 non-specialists. That's a lot easier to reach in the early game, both in terms of time to grow and health/happy cap.
I also always find it's faster to build the requisite library/NE in my GPFs if they're not flood-plain based, as well, just because I can start working hills faster.
Lets compare - 2 fish (+4 food after a lighthouse), and 1 wheat (+3 food if not irrigated). Add the city square (+2 food).
That gets us, at pop 9, 3 workers and 6 specialists, +1 food for a grass hill or growth. If we run 4 workers (1 on a grasslands farm), we can get 7 specialists at size 11.
The flood plain city has 31 food with 7 workers, meaning at size 15 it has 7 workers and 8 specialists. The extra workers are a lot of overhead.
I bet I can get the 3-food city to size 9-11 much faster than the flood plain city can get to size 15, even if I'm one specialist down by comparison. Of course, the size 10 city could work 2 grasslands farms to get to size 13 and 7 specialists, but I'd probably use surplus food/happy for production. It's far rarer to find a 3-food city, so perhaps a 2-food city is a better comparison, but I'd think a 2-food city is a decent GPF, not a tailor-made GPF. The 3-food GPF is also much more steady in the face of war weariness, because it's smaller and so more likely to be below your happy-caps, giving it room to stay working in a war. This is also nice because this step in the game is usually where my nearest enemy has built the SoZ.
The reason getting the GPF up quickly is important is that GPFs are largely useful to generate lots of scientists for a liberalism race, meaning they need to be up and running after you get literature/CoL, but before you get liberalism. That's a relatively narrow window if you're rushing to lib for a nationalism/gunpowder run or something similar.