If you aren't putting them on a food-giving tile, you are sacrificing food elsewhere to support the citizen working the GP improvement tile. So, for example, if you have a post-Civil Service riverside farm (4 food) and plop your GP on a plain desert tile, the excess food from the farm tiles is being used to support the citizen working the GP tile improvement. All things being equal, I would prefer to put the GP tile improvement on a tile that is food-self-sustaining, so I still get the excess food from the 4-food farm.
Best are tiles that don't involve sacrificing any food (grassland cows are best -- you lose the hammer from a pasture, but as long as you've already built a stable you get 3 food, 1 hammer and the GP tile yield). Next best, in my book, are food tiles whose farms do not enjoy an extra apple until Fertilizer, so the only loss is 1 food from a farm (and who really wants to work a 3-food grasslands farm?). Non-riverside grasslands are fine -- 2 food plus the GP tile yield. If I'm only left with 1 food tiles (non-riverside plains tiles, plains horses, sheep on a hill), then I'll usually drop on the horses first (lose 1 hammer from pasture) and then non-riverside plains tiles (forsaking 1 farm food until Fertilizer) or the sheep (lose 1 food from pasture).
Least attractive are worthless tiles, like tundra or, even worse, flat desert.