I almost always plant all of my GP in capital (or sometimes in holy city, if it is not my capital, but thats rare). The idea is that you capital is most likely your strongest city and thus you all buildings that give you bonus in % should go there (like School of Philosophy, etc). That means that you want to maximize both % and base yields there, thus all great people go in capital. This is ESPECIALLY important when playing Tourism, cause % bonus from Tourism is higher than for all other yields.Since it's on-topic, what are your rules to decide where to place your GP Improvements? I usually place them on Horses since Pastures are really weak improvements until much later on in the game (Fertilizer is the first tech that improves them I think) - Compared to Mines on Iron which get improved by the Forge then Steel early in the game. So I connect my horses with GP improvements - Any of them except Towns which must be on my trade routes, of course, and with Academies in my capital preferably (due to School of Philosophy, that always goes in my capital since the Palace gives science).
When I've ran out of horses I place them on Tundra tiles or isolated Grassland/Plains that wouldnt have made good farms.
I'm a bit new to Tourism but in my most recent game I've been placing my holy sites on a very nice coastal city, not my capital, where I'm planning to build trade route-related buildings + culture and send tons of cargo ships to a France across the sea that's fairly weak in culture.
As for what tiles to plant on - you want to plant them on resources, because you have a limited amount of tiles to work - you also want Specialists, etc. In the end my capital always works ~3-5 best farms or Fish, 7-8 GP tiles and the rest citizen are Specialists. I usually plant them on resources with weakest improvements first and slowly replace all resources with GP-tiles, so yes, Horses/Stone/Sheep/Cows go first. Of course you want Towns on roads, but i usually use them on WLTKD.